Bryston 4B-SST versus 3B-SST

Aside from obvious power and bass-control differences, what are the sonic contrasts between these two amps at lower playback levels , e.g. in the midrange/treble and in terms of imaging, soundstage, transparency, etc.
I have owned Brystom 4B SST and 14B SST amps. My experience with these amps has been that they display very similar sonic characteristics, with the 14B SST having a bit more "Fullness" at lower volumes. If you're anything like me, once you buy a Bryston amp, you're going to eventually wonder what more power will do for you in your system. I'd go for the 4B SST. More power, and better resale value (due to higher demand) seems to dictate this decision.
I 've never heard the SST's, but, I do remember prefering the 3BST to the 4BST. Damn'd if I can remember why though?