As an owner of Bryston 4B-ST and 5B-ST amps, and someone who has also done fairly extensive listening to the newer SST models, I have concluded that the SST models have somewhat different sonic characteristics than the ST models, but I'm not sure I'd go so far as to say they are "better". Some owners of Bryston amps, as well as several audio reviewers, have said that they prefer some characteristics of the ST line to the SST line, but it would take very good speakers and a discriminating ear to hear much difference between the ST and SST models.
The 6B-ST is a 3-channel amp that has the same power rating in each channel as the 4B-ST (it actually uses 3 of the same amplifier modules as found in the 4B-ST).
Unless you really need a LOT of power, in which case the 7B-ST monoblocks are a great choice (and an excellent buy at $2900), I'd probably opt for the 4B-SST and save some money for later upgrades. The 4B-SST is an excellent amp, and is rated very conservatively at 300 wpc (actual bench tests reveal that it could be rated at about 350 wpc).
The 6B-ST is a 3-channel amp that has the same power rating in each channel as the 4B-ST (it actually uses 3 of the same amplifier modules as found in the 4B-ST).
Unless you really need a LOT of power, in which case the 7B-ST monoblocks are a great choice (and an excellent buy at $2900), I'd probably opt for the 4B-SST and save some money for later upgrades. The 4B-SST is an excellent amp, and is rated very conservatively at 300 wpc (actual bench tests reveal that it could be rated at about 350 wpc).