Very large room HT

HT? Just stay with a high efficiency THX loud speaker system, follow the online setup parameter suggestions, and use audyssey or some DSP room calibration to help balance it out. Find the speaker system that fits in your budget, allowing the rest for filling in remainder of components. Go good pre/pro with room correction, and you can budget for 5.1 channel amp with power needed to driver your speakers. Cover reflection points on wall/ceiling points (use absorption up front, diffusion/absorption alternating to sides, and diffusion in back of room), bass traps in corners, and use even number seating (1/3,1/5,1/7 room dimension seating locations from side to side and back to front -and 1/6,1/10,1/14 corresponding loud speaker locations for monopoles, and dipoles to direct sides of rows) risers for back rows.
If your really at $20000 range, I'd suggest looking at Klipsch Thx, M&K POWERED THX speaker system, or like Macintosh THX system for more audiophile refined sounding qualities in an THX system. You're after high efficiency, focused verticle dispersion speakers designs, multiple mid/woof driver configured, even horns or multiple tweeter configs. Better power handling, more solid reinforcement of sound signal, less distortion (all things equal) and so on. Powered /active speakers also offer better power handling, higher efficiency, better dynamics and impact typically, yada yada.
There's a million combinations of speakers and equipment. I'd suggest looking at what all the professional AV magazine reviews use as their reference systems, and compare. They know what works, and are into this stuff. Take a page out of their book and follow their lead, yeah?

Monitor Audio Pl-200, PLC-150, PLW-15, GXFX, Pass Labs X250.5 ( update to 600.5 this year), Primare 30.5 mk2, ( centre I use bi-amp Red Wood, surroundspeakers I use Audioquest Rocket 33) Onkyo PR-SC5509 with Audyssey Pro ( I measure at totally different places and hight compared to Audyssey which gives me a wider and deeper stage, more drive and resolution. Sharper individual focus), Audioquest Redwood 2013 bi-wire for Pl-200, also for centre bi-amp version, use 2 channels of the Primare 30.5. Audioquest Sub3 ( in a few months update to the new Watchdog pure silver subcable from Audioquest.Acapella pure silver XLR Audioquest rocket 33 for surround speakers. Purist Audio 2013 LE powercable with Oyaide F1, Dominus rev b, 20 th Ann with Oyaide F1 3 of these. KE Powersource+ conditioner ( is also available in the US, is superior to all the PS audio ( I did test these),
Audioquest Sky for surr and centre ( upgrade to Audioquest wild blue Yonder wenn avaiable), 3 times Audioquest Diamond hdmi, Humax 5100 decoder, Onkyo BD-SP 809, Audioquest NRG1.5 for bluray player. Samsung PS64F8500 plasma

* Aurender S10 is also planned for this year.

I sold Klipsch speakers for some years. You can also play very loud with these. But in the world of highend, they are not good enough. Why? Very simple it has to do with there cross over filter. Not a lot of depth. What I said many times depth and also playing beside of the speakers is the most thrilling part in highend audio. Because all parts are all full loose and tochable from eachother in wide and in depth. You can hear all the parts of recordings played in full 3d in front of you. I alway say: you can run threw the recording during listening. So you can focus very easilly on one part and later focus on another part of the recording.

I disagree. I've had some of the most expensive high end audio gear and loud speakers imaginable in my systems, and played, installed, and tinkered with most all of it.
Here's a little snipped of the review of the Monitor Audio PL-200's:

"At very high levels, the PL200s couldn't match the sense of dynamic ease that characterizes the Avantgarde Uno Nanos, but the same thing might be said of just about any speaker of similar size and driver complement. "

Boys and girls, THIS IS TYPICAL OF MOST STANDARD "HI-END AUDIO" HOME AUDIO LOUDSPEAKER OFFERINGS, attempting dedicated home theater system duties!!!"
Yes, if large scale dynamics, effortless power and presence, and focused, solid imaging, coherent, acoustically forgiving designed loud speaker designs are not in your consideration (GO TO ANY IMAX THEATER AND TELL ME THE SYSTEM DOESN'T STOMP YOUR GUTS OUT AND SMACK YOU IN THE CHEST WITH DYNAMICS!!), than you you can mostly go get just about any "hi-fi" audio speaker, and stick em in your system! FOR MOVIES -especially - you can be sure you'll not get ANTHING much more special, from one choice to the next, that's ANYTHING MUCH MORE SPECIAL than what you can get from any other similar loudspeaker design! You don't think so?! YOU JUST READ FROM THE SAME REVIEWER WHAT THE SPEAKER DOES AND DOESN'T DO!
Let me tell you what they DON'T DO...ANY OF THE ABOVE NECESSARITY EFFECTIVE TRAITS THAT A GOOD HOME THEATER LOUDSPEAKER IN AN ALL OUT HT SYSTEM SHOULD OFFER! - that's what!
So if delicate, micro scale, 'LAID BACK" , to open sounding, and average HT dynamics, coherence, and presence for THX system duties is your cup of tea, then by all means. you can easily go with some OVER EXPENSIVE home audio music loudspeaker, and stick em in an dedicated home theater system!
I say, why waste your money, when you can do all the necessary HT soundtrack playback(movie recordings aren't Dianna Krall and high rez super audio cd master 2 ch recordings, btw) with absolute effectiveness WITH A TRIED AND TRUE INDUSTRY RECOMMENDED THX SPEAKER SYSTEM for your EXTENSIVELY DIFFICULT ACCOUSTICALLY UNFRIENDLY DEDICATED THEATER ROOM SETUP, which likely doesn't work so effectively with standard music speakers in the first place?!!!
If you're going to go the Thiel, Wilson, B&W, esoteric music kit choices for your DEDICATED home theater setup (not music, btw, as was not described here as an application), then simply save your money, and buy most any highly rated Class B or A Stereophile monitor on the used market, add a subwoofer, hook it all up with your choice of processor and used multi-channel Class A or B Stereophile recommended amplifier and Oppo player, and be done with it!
I assure you, ALL THE HIGH END AV AND EVEN STEREO MAGAZINE PROFESSIONAL REVIEWERS are using more dedicated HT type home theater loud speakers for their "dedicated" HT systems!
Not what's being recommended previously.
You're choice. But it's all been done before
Yawwwwwn.
The Klipsch THX system still is WAY more effective, "CLEAR ENOUGH", detailed "enough", and super coherent, dynamic (for a passive speaker system), MORE than potent and hard hitting enough, and solid enough of an imager (no you don't need super super deep imaging from an ht loudspeaker, YOU NEED PRESSENCE! -even slightly upfront soundstaging would be preferred over laid back audiophile!) to be a better choice for most applications of this type!
Yep, to many years of tinkering with esoteric audio gear to know what to recommend, what works, and what doesn't.
I guarantee NO ONE with what I recommended will be disappointed (except the 2 channel tube/analog only guys listening to "jazz at the Pawn Shop" on a loop, will think ill of said recommended THX's formula! - 1000's of hours of tried and true home theater select gear, btw!)
The used listing for speakers is full of choices that offer nothing so special, soundwise, if you're main goal is an effective, all out home theater speaker system.
Just sayin...

You have to read better. Before you talk this kind of bullshit. The Pl-200 is not a big speaker, so it does not go that low. But........it can play till about 118 db without stress. If you had better read my threats you would have known that I use the PLW-15. This sub starts at 16 hz. With my way of measurment and Audyssey Pro it get the best stealth integration ever with a subwoofer. I was at a show with over 50 distributers. At the show were; the new Dynaudio evidence platinum, Focal Scala's, Magico S1, new 800D and many more. Many distributers came to me to congratulate with the best sound of the show. The day after the show the chief editor of the biggest Audio Magazine called to say we were the absolute sound. And you know why......Because I had the best timing, the most convincing 3-dimensional image, best natural sound. On monday many shop owners came to me thay they never heard this level in touchable sound ever. And now back to Klipsch. The difference between the PL-200 and the Klipsch are the cross overs. The stage of the Pl-200 is wider and deeper. This part I can use to the max. I play 5 metres behind the speakers and 1 metre beside. This makes the stage so much bigger. The instruments and voices are foccussed sharp as in real. Most people never heard a demo what was this touchable. I played many bluray's at live level at the show. Klipsch was also at this show, I did not hear any person talking about how great Klisch was. Klipsch has not the accuracy in playing this deep and wide as the best highend speakers do. The stage is less deep and less wide. Instruments are not that loose from eachother. Playing loud does not say anything about the stage it can build. This is the part were you can reach the highest level. I could play extreme loud but still with a very deep and wide stage. No harshness and the sharpest individual focus of instruments and voices most peole ever heard. Next time think sharper before you think your Klipsch speakers are that special. Because in depth and in wide the crossovers are not that special. You never can reach the 3-dimensional level in sound. This part is the reason to blow every person away.