Bryston 4B-SST vs. 6B-SST

Within the next month or so I plan on upgrading my two channel amp, currently a 4B-ST. You can see from my "system" link that I have a home theatre setup, but, I primarily listen to 2 channel.

I currently have the 4B-ST on the front two channels and a Rotel RMB-1075 (5x120) bi-amping the center and a single channel to each rear. This of course leaves one channel unused.

I like the way the 4B-ST sounds, I really like how Pass Labs (X250) sounds but, budget limitations have nipped that in the bud, sigh. I do like the newer Bryston SST's a lot as well, and they are affordable to boot.

I have been able to audition a 4B-SST but not a 6B-SST. The specs are the same except for the 6B being 3 channels instead of 2. They do look different internally. The 6B looks modular while the 4B does not. It would work well if I could use the 6B-SST for the fronts and center and swap my current 4B-ST to the rears.

Has anyone compared these amps sound qualities, especially with 2 channel?

Do you think, or know, of any reason that would make the 3 channel amp perform less suitably in a 2channel situation?

Any thoughts are greatly appreciated.
My thoughts were, that the modular design of the 6B-SST may limit its abilities in comparison to the 4B-SST. I presume they have similar sonic characteristics, but since I have never heard the 6B-SST I thought it would be better to ask.
I'm looking at the same setup; the 6B-SST is currently my preferred solution, but Theta's Dreadnaugth II is now on the radar screen, as well. My own view is that at this price point, and level of quality, much is determined by 'what name you want on your system, rather than true performance differences.
Funny, I find myself in the same exact boat. I have considered the Theta Dreadnaught II and both Bryston units. I have only 2 front speakers now, but want to get into HT fairly soon. There are plusses and minuses for each situation. The Theta can be purchased as a 2 channel and additional channels can be added later. Theta doesn't "punish" you for buying modules later rather than at time of purchase. The modular Bryston, the 9BSST I think, has a steep price increase for modules purchased after the original sale. The Theta can also use 225watt modules or 100x2 watt modules. You could conceivable run 10 full channels if you wished. I am thinking of getting it with 2 225's initially then adding as I grow.

Converesely, the Bryston is a bit more of a dillemma. If I go with the 4BSST first, I will have the amp for my current system. Adding a 6BSST would allow a quicker additon of center channel when the funds for the speaker come. But going the 6BSST first will require me to wait much longer as the price difference is considerable.

I would love to hear what others have done in similar situations. Ultimately, the Brystons will end up being about $1k more than the Theta for 5 channels but, with the 20 year warranty, I think this is more than worth it. Guess I will have to do some serious auditioning and let my ears choose.
I havent made a decision yet. Logic is pushing me towards the two-channel amp. The foundation of my logic is, since most of my listening is two channel, a two channel amp will be the most efficient means to effect change now and down the road. I know I cant afford a big Pass right now, but I can wait and save. If I lock myself in with a three or more channel amp, then future amp changes will become more costly and technically more difficult. I suppose a modular approach to system building is what you can call it.

The caveat is, that the center channel amplification will probably be different than the L/R fronts. That is what was pushing me towards the 3 channel configuration in the first place.

I am glad you guys brought up the Theta. FWIW I have read at least a few comments on how great the Theta sounds. I am going to look into that possibility. In all honesty though, my mind keeps drifting back to how good that Pass X250 sounded. I wonder If the Pass has good synergy with the Bryston Processor (SP-1.7).
When my 6B SST was working it sounded great on 2 channel and surround. I even got the impression that playing 2 channel was like having 2 mono amps. The SP 1.7 works fine with my Coda Amps....if thats a help. But since I have gone back to using Coda I decided to go with a 2 channel and a mono for center. Both amps are matched output. The only thing I noticed on the 6B SST is the 80lbs! I am not a true audiophile and have not heard a lot of the gear you guys have. I just jumped into vinyl with a Music Hall MMF-7. I love it. But it's a lot of trouble compared to a cd.
For an update, I decided on the 4BSST. I made a few offers on used Pass X250s but no takers. The Pass is so deep, physically, that a new rack would be necessary. Surprisingly the SST isnt killed by the Pass, in fact its a pretty close race.

I decided to go with a 2 channel and a 5 channel Amp. The 2 for the stereo mains and the 5 for center and surrounds. I can biamp the center, run the two surrounds, and still have one channel left over for the rear center (6.1 setup). Plus I want the 2 channel path discrete for future upgrades.

All this is helped along by my loss of enthusiasm for multi channel hi-rez. I dont have, nor have heard, a single hi-rez disc in which I preferred multi channel over two channel. In fact, my modest Jolida CDP usually sounds better with the hybrid layer! Maybe that will change when funds become available for a source upgrade. Though redbook will be my main focus.

Secondarily I want to have some Vinyl experiments, which has my Wife all excited, BTW. When she figures out what that really means I'll probably have to take her shoe shopping or something.
I agree with you on the 2 channel part for stereo play back. Thats what I like about the most on my Sp 1.7. I have a great 2 channel preamp. I went to vinyl for 2 reasons. DVD and SACD selection. Plus I have gotten great NEW lp's off ebay cheap! I mean 6 bucks delivered! I even got 30 NEW old LP's for 14.95. I am only buyibg new LPs so I don't have to invest in record cleaning machine now. I'm very impressed with vinyl. I also decided to stay with my 4 channel amp for suround so my main amp will drive stereo. Buying a mono amp for 2500 bucks leaves me more money to put else where at this time.