I'm getting ready to order the 990. Here's my question.
I want to set my mains to large, center to small, surrounds to small, but.....my rear surrounds to large! Can it be done?? I do have my reasons and I will be glad to elaborate if anyone is curious. But can it be done??? Thanks!!

Nope, I'm afraid the 990 will not allow the rear surrounds to be large if the side surrounds are small. Rear surrounds can be large if the side surrounds are also large, for what that's worth.

I'm curious about what setup would lend itself to large surround back speakers - I'd expect that most sources aren't going to give you much (if any) content in the lower octave bands anyway.

Gonk! As far as LF in the rear surrounds are concerned...you'd be surprised! Of course, various soundtracks have across the board responses with DTS 6.1 tracks the best.(The scene in "Return of the King" in which the King of the dead laughs at Aragon's proposal is an excellent example).
But none of this matters unless you have lots and lots of room space. My rear surrounds are approx. 13-14 ft behind your head. My faux 7.1(2) system is configured with a NHT SW-10 subwoofer hi-passed to M&K 55 Tripoles as the rear-surround component. I outboard an ancient Denon 3802 to my amplifiers(ATI2505, Elite M-90, and on occasion, some outlaw 200 monoblocks).
When the sound sources are a good deal behind your head, you start running into the "Hall/Precedence effect" concerning spatial cueing/location. With a dedicated 'rear' subwoofer,(my main sub is a M&K 350), that is properly blended with the satellites, soundtracks with rear/Lf are convincing and realistic. A lot of this has to do with the Munson/Fletcher response curve that gets wildly skewed with audio cues coming from directly behind. Anyways, I could go on a lot more about this, but suffice it to say I use this configuration with 6.1 soundtracks mostly and certain D/D soundtracks of merit(think Star Wars!) However, for the majority of soundtracks I actually prefer 5.1 for clarity that suffers from "matrixing" when expanded for rear surrounds. I don't know of anyone with this kind of set-up...but trust me....the LF in rear surrounds is more plentiful than most suspect. There is no processor for any money that will let me have my pre-eaten cake..My ultimate solution will be LFM-1(s) for every channel! 7.7 here we come!!!!!(just kidding)

Here's one thought - let the surround back channels stay small, but set their crossover all the way down to 40Hz. Since there's a quad crossover, the side surrounds can stay at 80Hz or whatever point works best for you. You'll end up with the really deep stuff re-directed, but you can still use the NHT sub and tripoles together as the rear surround crew without starving the NHT. In fact, the 10" sub may appreciate handing off the very bottom of the spectrum to the M&K 350.

Oh honorable mayor of "Outlawville"! I went out for a cigar and had almost the same thought as you did. You're quick and you do really understand...
Anyways, I"ll wake up tomorrow @ six o'clock a.m. and place my order...DHL overnight so I can scoop up the 990 and take it up to the mountain home(6 k-ft up baby!) I'll feedback my impressions to you and all the boarders sometime next week. Thanks Gonk for the reply and I hope to join all in Outlaw bliss!

Could you explain this a bit more? Are you localizing low frequencies or hearing certain sounds before others? When you say "Hall/Precedence effect" do you mean the Haas/Precedence effect? Are you talking about Fletcher-Munson loudness compensation curves? Or is there some other research they did regarding localization?

Thanx.