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.
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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?