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)
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.