I can't weigh in on the difference between a 9B-SST and a 6B-SST squared, but I recently moved from a 9B-SST to (5) PP300's. I may be incorrect in my understanding, but unless you are using the balanced input on the 6B squared, you will not be getting the "full" effect of the squared series. Moreover, I am not certain if the 6B gets the "full Monty" squared treatment since it and the PP300 are based on essentially the same circuit topology and from what I understand that design does not lend itself to the full complement of features that the other amplifiers enjoy with the squared update - particularly the 7B, 14B, and 28B.
I noticed most of the difference in effortless dynamics and a soundstage that was a little more forward with the PP300's as opposed to the 9B-SST. I am running the Gallo Reference 3.1s, which are much less sensitive than I believe Mr Gallo claims, and was clipping the 9B's channels with pretty moderate sound pressure levels which brought about the desire for the upgrade. I preferred the sound of the PP300's to the 4B-SST - slightly more width and depth - but made my purchase just before the squared series began shipping. I have not yet taken the time to bring a squared version home to audition it in MY room but will shortly - perhaps when my tax refund check arrives. aa
In terms of surround power requirements I don't agree with the all the "experts" that you need to have the same power. Theoretically you could have a soundtrack that is mixed that has that much power requirement for the surround channels but it has been my experience that with 98%, or more, mixes required much less, particularly if you aren't running full range.
While I was waiting for the rear pair of PP300's to arrive, I inserted my back up 2B-LP for the surround channels and didn't once have them go into clipping during any movie I ran. No doubt someone will come up with a short list of movies that place great demands on the surround channels for one or two effects, but unless cost isn't a consideration, you can get by with much less IMHO. Having said that, most agree that the most coherent soundfield is created with 5 (or more) identical speakers being powered by 5 (or more) identical channels of amplification.
The center channel is another matter and I wouldn't skimp here. Ultimately I will end up with a 14B-SST sq or a pair of 7B SST sq for the stereo channels and run a pair of PP300's on the center with one for the bass driver with the surrounds having a PP300 each.