Best HT speaker system

B&W makes two classes of matched systems which includes the units that hang on the wall at the mid-point and/or rear. One is in the range of the 600/CDM series, the other is mulitple-thousands and in the class with the Nautilus speakers. They are both THX certified. See the web site which is BWSPEAKERS.COM

I guess there are four B&W classes in all Nautilus, CDM, 600, 300; All but the 300 series have a wall unit which would give you a 7 channel system if you used them at the mid-point and still had rears. If you really want the very best money no object the Nautilus line are excellent. The CDM-NTs are quite good also. I would consider using 7 speakers if you really want the best experience, and you may well see Dolby 7.1 decoders sooner than you think.

For HT I've become a fan of Aerial loudspeakers. They play loud and clean with good punch, and will provide excellent treble without getting harsh. You could run up front LR3/CC3B/LR3 (2x7", 1x5", 1x1" driver configuration), and for the sides and rears a combination of SR3s or LR3s. Not cheap at 1.5k each. If your budget allowed Aerial's new LR5/CC5/LR5 up front would easily pressurize your large room, but you may need more power.

Go Talon Audio speakers. They are among the best. If money were no object, I'd get the Talon Khoruses for front and rears, the Peregrine for the center channel and two ROC subwoofers. MSRP about $40k

I had considered using Kef model 4's as fronts, w/ model 2's in the rears, but my space is just too small. You have plenty of room, but make sure you are on very solid ground,(preferrably concrete) because these speakers can put out a lot of energy. A sub may not even be necessary, depending on your tastes. But if you want to wake the neighbors then by all means add a sub.