Adding Home Theater To 2 Channel System

I have a two channel system that I am very happy with located in a dedicated listening room.  I am considering how to add home theater without giving up the two channel sound that I enjoy.  My main speakers are Avantgarde UNO’s (horn speakers - 18 ohm, 108 db sensitivity). Avantgarde does not make a center channel speaker.  I assume that I will have a problem matching any box speaker center channel speaker with the Avantgarde - my worry is incoherent sound.  Is this something I should worry about?  Are there center channel speakers that might be a good match (I have been unable to find any obvious candidates ohm/sensitivity match)?  Is it likely that I will need to find a new set of front speakers from the same brand as the center speaker?

The rest of my setup: Pass Labs XA 60.8, Audio Research Ref 6 preamp, SME 20/2 turntable, Lumin x1 streamer, Ayon CD player, REL 812/S subs (2)
Don't do it. Doesn't your stereo image well? So what do you need a center channel for? Don't you have a wide expansive sound stage? The worst thing you can do to a fine system like yours is add a surround processor, of any kind. Any price. Does not matter. Instant 20% drop in system sound quality, even when listening to music in stereo. The movie sound field will be worse than in stereo. All you gain is some cheesy surround effects, that never will be anything like the holographic seamless organic world you can get from stereo.  

Not for lack of trying. I tried nearly two years. Two wasted years. I say wasted, because instead of anyone learning from this they all go and repeat the same mistakes. Build a whole new room if you must. Put a screen between your speakers. Anything but add HT dreck to an otherwise perfectly good stereo.
It can be done BUT you need a preamp with bypass that puts the processor out of the loop for 2 channel, a good pre pro will have room correction dirac or some such that will MOSTLY take care of sound mismatch between speakers
This is very easy to do. I have been doing 6.1 (7.1 with a phantom center). 
Millercarbon is right. You do not need a center and they all suck (for the most part) anyway. Surrounds and subs are needed for sure. 

You just need a receiver which can be the cheapest one you can find with preouts for the left and right front and you send the preouts to you two channel system. A home theater bypass is a nice feature but not needed. 
If you don’t have a bypass you can just use an open analog input and set the volume to the same spot every time on the two channel preamp.  When you watch a movie you control the volume through the receiver. At that point you just hook everything up as normal. 

I personally would not spend a lot on rear speakers. I use 4 Klipsch RP8000f in piano black for my rears and just change put my fronts to what ever as normal for my two channel hobby. Another cheap option if you don’t mind the look are JBl 590 or 580s for rear speakers. 2 of them would work well and are super cheap with a horn tweeter. JBL blows them out for 1/2 price twice a year or more.  I have seen 590s go for $900 a pair and honestly they are pretty good speakers. 
Even though the klipsch are 100% different from my Thiels up front I have zero issues with a miss matched sound front to back. It just does not matter. 

Subs are critical to home theater. I have two JL audio E112 subs and use them for both two channel and home theater at the same time. They have two inputs each and those inputs are summed. So the two channel preamp is run to each sub and the the LFE low frequency effect signal from the receiver is run to the other open input on the sub. 
This allows the subs to work with the high pass crossover for two channel and I can still adjust the gain of the sub through the receiver for movie sound tracks. 

If you put it on the front wall and leave your speakers where they are you will never hear it. Honestly they are thin and soft so they are hard to hear between your speakers imo anyway. Screen innovations makes a nice hanging system you can hand from the ceiling too. I hang mine about 3’ off the rear wall and have my speakers 6’ out into room as a compromise for both screen size and viewing angles for the other seats…

but go big!  At 12’ Away I would want at least a 130” screen. 
That system will probably sound better in Stereo than trying g to hodge podge a HT around it....   I used to have a preamp with a HT pass through and ditched the 5.1 for 2 .1 and never looked back
home theater gear will have zero effect your two channel. They can run 100% independent of each other.
A 4.1 setup is so much better for movies than 2 channel. Much more involving. Speaker placement is key just like two channel it makes the biggest difference.
millercarbon is right, a good system does not need a center channel. Doesn't need rear channels either. It will need killer subwoofers.
I also have a 2 channel theater. Check out my system page and you can see it. Just hang a screen between your speakers and you are good to go. Not quite. It took me several weeks to get all of the rattles and buzzing out of the screen and it is a Stewart!
I'd replicate the advice/sentiment shared already and forego a center channel. If ever that route would have to be taken the only true solution would be 3 equal front speakers, period, but then you'd need a perforated screen + projector for that to make any sense. 

What I would do instead, and that wouldn't only serve Home Theater duties, is investing in a massively capable sub set-up, which is to say loads of displacement, headroom and no less than 20-25Hz extension (some may say honest 15Hz is required for HT to cover most, as in 99% of the infra-sonics found on the many Blu-ray/4K UHD titles, but a lower tune impacts the overall sensation of the bass - i.e. central to upper bass, and hereby integration in particular - so it's a compromise). This is MANDATORY for movies, but properly integrated (which goes without saying) will serve music reproduction as well. 

As for the particular sub solution (and ULF capabilities) to choose here, I'd forget about über-expensive subs and instead amp up their number for a DBA sub set-up, or go with 2 very large ones. If acoustic live performances are very much your cup of tea and form a reference for your home audio reproduction, I'd place the subs (say, 2 or 4 of them) symmetrical to the mains and coupled in stereo. If not a mono-coupled DBA placed asymmetrically may float your boat. Either way, never skimp on displacement and headroom, and get sufficient extension.  
Thanks to everyone for the terrific and consistent feedback.  It will save me a lot of time, money and disappointment.  Your advice on this is invaluable.
Good to hear. 
If you end up trying rears again just use any old speaker you have sitting around or get something cheap.

 timbral matching the rears is a waist of money unless you are really into it. 

Highend rears is a waist too. If you run high end rear speakers you will feel obligated to power them right and use good wire etc, and it all ends up not being worth it. I think for 99.9% of us we would rather spend that money improving our two channel. 
A screen is the best way to go. If you really want the least possible compromise use an acoustically transparent screen, one designed to have speakers behind it. This way you can put whatever acoustic panel you want behind the screen and it will still work. Or for more money a powered screen you can raise and lower, but those come with their own set of problems. 

In any case, done right you will wind up with four subs in a DBA and this alone will elevate your stereo like you won't believe.
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You may find this link helpful for screens. I use a 100”, tab tensioned model that rises from behind and at the level of my front cabinet. When down, it can’t be seen. 15 minutes to mount the 2 wall brackets and use with an ultra short throw projector and it’s an easy set up to HT. use full range Voxativ widebanders  the 2 channel and matching doesn’t exist. As already mentioned above, Klipsch sounds close as they use horns and are closer than most brands to matching the efficiency of the Unos and the Vox. Have fun. 

I also have a parasound preamp with bypass for Home Theater.  I disagree with opinions about the center channel, which is important for dialog. I have a dedicated amp for the center channel and KEF LS50 speaker.
OP -

If you want a true home theater experience it’s easily doable and you won’t duplicate it with a 2 channel set up no matter how hard you try.  That said, if you have a great sub set up integrated with your 2 channel set up, you may be really happy with it for movie / TV watching. Would it match a true Home Theater set up, nah, not even close. 
Are the comments on a center channel correct, they are.  Truth is a high percentage of a movie soundtrack mix is played through the center channel, mostly due to dialogue dominating the mix.  Center channels are flawed inherently as they were designed to fit a set up without being in the way of the screen. All that said, most of what the center channel will play is dialogue - a decent center will do all you need, it won’t be anywhere near the same league as a speaker you’d use for your 2 channel listening, but it will do the job intended and do it well. This assumes the Center is used in a true Home Theater set up.  If you went the route of 2 fronts and a center only, everyone is absolutely correct that you should skip the center.
The invention of HT Bypass and really stellar HT room correction will blend mismatched speakers and even amplification extremely well.  Adding gear the right way will have zero impact on your 2 channel set up.  In fact, you won’t even have to turn on your Home Theater Receiver / Processor.  I love both, I wouldn’t give up either when the gear / Technolodgy exists to deliver both with little to no compromise.  Again, if you integrate HT Bypass correctly, keep your beloved 2 channel speakers, when you listen to 2 channel, you never even turn on your Home Theater gear, how does that impact your 2 channel…. The only tricky part can be sub integration with both, but that’s easily tackled if you have subs that allow you to save multiple set ups, one for movies, one for music.

I’ve had 5 Home Theater processor’s / receivers, none have come close to a dedicated 2 channel set up for music, either a pre / amp set up or integrated set up.  That said, running 2 channel with Subs doesn’t hold a candle to the movie watching experience with a HT set up.  If you love that part of the movie watching experience you owe it to yourself to have both, period. Sharing the same room, front speakers is more than doable.
Most that participate in this forum are heavily focused on 2 channel HiFi and that makes perfect sense.  Home Theater for most is a distant second and for some reason is always seen as a massive threat to a 2 channel set up.  Very similar to back in the day when some 2 channel purest would avoid sub woofer integration with 2 channel at all costs. If you went on a Hime Theater / AV forum, you’d get a heavy lean toward HT.  I happen to love both and share the same passion for both equally, seems to be unique as most really lean one way or the other. 
I am using a Classe Sigma processor  that I can put in digital bypass when playing 2 channel music with my Lumin T2 streamer. In bypass mode, the full analog signal bypasses all processing and the Classe acts as a good analog preamp.  Gives me the best of both worlds.
The topic was was discussed pretty well before. Build separate, independent system with own speakers for Home Theater. 
Yes, in a perfect world, 2 separate systems is the way to go. For some people, this is not always practical due to cost and/or space issues, I am a 2 channel guy, but love listening to my many Blu-ray concerts that sound much more spacious in 5.1
Perhaps think about getting an ARCAM AVR 850 and adding a center channel speaker and two rear surround speakers.  Whey you switch to two channel there is plenty of clean power as the first 50 watts are pure A.
I solved this problem by positioning TWO centre channel speakers next to the TV, between the main L and R speakers. Those two speakers were smaller versions of the L and R mains. Two more small speakers for surround speakers, and I had HT when I wanted it and two channel when I wanted that.

Those two smaller speakers above and below the TV is an even better solution if that can be made to work. I moved to that before I moved to separate dedicated HT and 2 channel rooms, and it's better. But a compromise is possible too. 
You’ve gotten all the advice there is so far. I will add mine. You have a highly evolved stereo. To add video to your room, make sure you don’t upset the acoustics you are currently enjoying.  MC is spot on in his observations. No Center speaker will blend nor is one needed. Add the Swarm. Get a 7.2 channel AV receiver for HDMI switching and to generate rear output channels, and get a pair of wide dispersion speakers for the surround channels. Ohm Acoustics might be ideal. Rig your own HT Bypass by finding the unity gain position on your SP6.  Relax and enjoy the best of both. Run your sub array off your stereo preamp, and set the receiver mains to large with + LFE. 

@bwguy. OP has dedicated listening room so I think the space is not an issue for him. The cost of home theater staff usually is not so high. Advantage of separate system for HT is pretty obvious. All in one compromise 2 channel and you don’t want your littering experience to be stigmatised by that. 
I for one like listen to music in the home theater room. I often game (online multi player) with the sound off and music on. 
 I am all for a combined system. You will not lose anything. Just gain experiences. 
I agree with MC. Don't do it. I did it in reverse. I had a dedicated home theater set up and I tried to integrate a two channel system into it. Miserable failure... lifeless! The answer was to have both, a dedicated home theater system and a dedicated two channel system separate from each other. I now enjoy the virtues of both for their intended purpose and I am much happier with the results.

I love the idea of having two separate systems, one for music and one for HT.  That said, I am having a hard time overcoming the amount of equipment clutter this will create ... 5 speakers, more subs, receiver, interconnects, cables.  I guess that might just be a “me” problem.

I already have 2 REL 812/S subs.  Adding two more is something I was considering just for music, nice that it makes sense for HT as well.  REL is very clear that their subs should be connected to the high pass on the sub and from there directly to the amp for music … no connection to the preamp.  This is the way I have set them up.  I guess/assume that for HT, the connection to the subs should be to the .1 LFE and then to a receiver(?). 
Tried it and it was one compromise after another. I ended up just adding an entirely separate system for the ht piece. Two channel has been kept as is with no connection to the ht.
Wow, This got a lot of responses. Just like when someone thanked MC for something and all hell broke loose. JK. I think MC is funny as hell. You are correct about the REL LFE input. You can use the speakon or RCA/XLR connection for the two channel and set up the L.F.E. Input for HT which has its own level control. So you are good on subs down to 28hz. Set the level at 10:00 position instead of 12:00 per the directions during the setup mode so you can add a little more when finished. Also, change the subwoofer distance in the Denon virtual setup by adding 10’ to what it truly is. Actual 15’ from you becomes 25’. You will be amazed. 
Denon made a model called the X3600H and then the X3700H which went to 8K but more importantly to 120 refresh rate at 4K. The amazing benefits besides the video is that you can divert the two front channel amps and use them for surrounds if you have your own front amplifiers like your two channel system. Good on the wallet too. 
When listening to surround sound 65% of the sound comes out of the center channel, 25% comes out of the fronts, and 10% comes out of the rears. You can have a fantom center if your two channel is killer.
I have played a full movie for people on just my two front speakers. My front speakers also go down to 25hz. It can be done. You might want to start with the surround receiver and some surrounds and go from there. Just remember the more speakers you add the more you have to control the sound. 11-13 speakers is a lot of sound to control. I say 5.? Channels unless you have two rows of seats then you go to 7.? to fill in the gap.  No TVs unless you will cover it up when listening to music. The glass is a sound killer. Another ideas is a virtual frame screen painted on the wall and a projector of your choice. 
Look at the post above for screen projector ideas. I do like the drop down acoustic transparent screen. It lets you go big. I might do this next. Be careful with Klipsch/JBL speakers as a match. They can dip down pretty low in the ohms department. Even below 4ohms. I say go with 8 ohm speakers for the center and surrounds and don’t worry about matching the horns. You might want to look at KEF speakers. The Q driver system works very well at even dispersion. I have seen their new $600 Q150 speakers which would work great for surrounds go for $300 a pair. Cross them over at 110hz and let the subs do the rest. What most people do not realize is the amount of DSP a surround receiver has will make a silk dome sound like a horn speaker. I have a Sonus Faber Cremona center with a silk dome tweeter matched with my aluminum tweeters on my mains. They sound like one huge speaker which is what you want. By using less amplifiers in the receiver you actually get more watt output to each speaker. 5 speakers pushed vs. 9 speakers pushed can give you 25 more watts per channel. Later you can get a dedicated home theater sub or room shaker that will go past 20hz and keep your RELs just for the two channel. HSU or SVS are good for that but I prefer JL Audio. Finally, Upgrade the power cord on the surround sound receiver. It makes a huge difference in the sound and performance. I like the Nordost Blue Heaven or Puritan Classic Plus. Seriously, Amazed! Sorry for the long post but this can get expensive and you can make a lot of mistakes. Start with, Denon-$1100, screen- $1000, projector-$$? and surrounds-$300 and build from there. Better yet look on Craigslist or AGon for a cheap surround speakers. 
I'm in the two separate system camp
In my credenza l have my VAC tube amp/ARC tube pre amp and my Emotiva XMC-2 16 channel!! Processor & Anthem MCA-5 that I've owned forever  I have two set of main speakers, center speaker and two SVS Sub woofers My rear speakers are wireless  l just turn on the system that l want to listen too 
You can do without a center channel but HT is carefully developed to have dialog specifically coming from a center. Because the frequencies sent to the center channel are relatively easy to produce so you can go with a very different design of speaker for center.
And quality or price is not critical-within reason.
A very interesting choice would be the MBL 111 single speaker currently for sale on AG. I think that is the model #. It is the small entry level one. It has been for sale for quite a while for about $2500.
Surely he should take a decent offer. That would be a super interesting center to combine with your Unos!
A completely separate system except for the Unos is best.
I don’t think it would be a problem to run additional speaker cables so that the Unos would have two pair of cables each.
Certainly, your choice of TH components depends on your wallet.
I have seen HT done with tubes but SS is most common.
Most important is to have the same amp for the RL and C. Best is a 5 channel amp and a good processor. What I would get is an older top line Meridian processor. You can pick one up for a fraction of their original price. Features will be more than you will need.
And getting an older pair of Meridian powered speakers would be great. Then you will need a 3 channel amp or 3 mono blocks.
I would get (unless you already have on) an OLED TV. Sony is best.
At least 65” but I would get a 75”. Going with projector and screen is tricky and probably double or more. A projector will give you about a 100” picture but not necessarily a sharper image.
In any HT, the processor is the most important and what you build your system around. For SQ and functionality, I love the Meridian. Especially since the older ones are still perfect for a basic 5.2 system. Think your subs can also take double cables but I think a Meridian also has two rca sub outs. I just want to emphasize that an older processor’s features will be all you need and more. The top line one is great but a G65 one can be had for about $1K (think the MSRP was over $5K!). It is also excellent. Mostly less features that you will not need.

HT is also designed to be backwards compatible. That is just how it works. 
I have done this quite successfully.  I'm not saying this is the only way, it is just the way I accomplished it.

First, I agree with the statement that a centre speaker can mess things up. I tested a decent one (Vandersteen) and it added absolutely nothing to what my main speakers could do as a phantom centre.  Do your own test, though. Some main speakers don't do as well at creating that phantom image.

I picked up a decent AV preamp and a separate 5 channel power amp for the surrounds. The only commonality between my audio and video systems are that the main speakers are used for both purposes. 

If I want to switch from audio to video, all I have to do is  pull the speaker leads for the main speakers out of the one power amp and plug them into the other power amp, switch on the appropriate preamp and go.

I also have a pair of powered subs that are used only for video use.

It may seem like a lot of fuss, but it is the only way to accomplish really good sound in the audio system for me. There may be an AV preamp out there that sounds as good as my audio preamp/amp, but I haven't found one, and my audio system power amp at 70 wpc wouldn't run at video volumes easily.

Basically start with a good audio system and add on the pre/power and surrounds as a stand alone that takes about two minutes to switch to..

A lot more response here than I expected.  Thanks so much to everyone for helping me with this.  Reading over the advice here and talking with others I think I will get into the HT pool in the shallow end and build from there as I learn or as/if needed.  That would have me start with a receiver (for the surrounds), drop down screen, projector and two surrounds.  This gets me in the game.  Again, thanks so much!
I did that about fifteen years ago.  The main thing is: ditch the center channel.  If you have really good speakers and placement, keep them and bridge the R/L speakers in lieu of a center channel.  You stereo will stay perfect that way, and you'll hardly miss a center channel (certainly better than a mismatched one).  Add the rear speakers and essentially run a quad system. 

This lesson did not come easily.  I started with five full range Thiel speakers .... the sound quality was superb .... the stereo imaging sucked until I removed the center speaker and experimented with bridging, which I could do either in the Oppo or in the surround preamp.  The main problem is that surround is mixed to a set speaker placement that has the L/R speakers much further apart and forward of the center to get decent surround imaging.  It sucks for stereo.  If you keep a center but put the L/R in their normal stereo position, you get a cramped front soundstage and slightly uneven side imaging ... more front/back than  it should be.

I've never seen this recommended anywhere but believe me it is real.  I found the same thing in three separate rooms as well as with alternative speakers.  Try it for yourself.  You'll find I'm right.