Very large room HT

7.1 does not say anything about the quality of speakers, amps etc. Wenn you use an amp which give a 2-dimensional image ( most do) you never will go higher than standard audio. Also wenn I play in Multi channel the stage is superior to all the 2-dimensional brands. I play 4-5 metre behind the speakers. And 1 metre beside the speakers. Bluray music is stunning because every instrument plays full touchable and to point out. Often instruments play far beside the speakers. For many it is a new experience. It is that convincing that people understand the new level in sound.

Thanks for your advice!
I realize that I need a good multi Chanel amplifier,
And preamp,to drive a 5.1 set of B&W with 804 as rt and lt?

First choose your personal speakers. And please do not forget B&kW make fine speakers but they lack a deep and wide stage. This is the most exiting part in highend audio. I did not go for the B&W 800D. I wanted to go to the next level for the absolute sound. B&W could not help me anymore because the stage was not that wide and deep. A 3-dimensional sound is wenn you see the biggest smiles on people there faces. I hope to give demo's in the US as well. Because I want people to hear how much more thrilling 3-dimensional sound is over 2-dimensional sound. Many people came to me during the show that they did understand what I meant with 3-dimensional sound after hearing the demo. It was new for many, even for shop owners. For me it is normal. I started in music and audio wenn I was 6. I got a big Philips stereo tube recorder.

Dear Bo
Can you detail your system?

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?