Bryston gear on E-bay is usually gone in no time at all, even very old gear is in demand. You might experience better sound with the SST models, but I think to some people its a "is it really worth the price difference" question. I say this mostly because here in Europe Bryston is very expensive indeed.
If I have to shoot from the hip, and I am not familiar with the PSAudio 4,6, so I cant vouch for that, but yes, Dynaudio needs juice, and the more the better really, like my speakers. If you tend to not play lod I think the 2BSST is enough for you, and I can guarantee a fantastic experience. James Tanner use the 2BSST with his Quad 2905 speakers actually. However, did you consider the B100?
(But dennis, you remember correctly about the comment on the 4B vs. the B100, I remember now....)