I recieved a later 3B ST THX (not SST, final checkout 2003/02/07) on a warranty issue (Thank you Bryston) which replaced my 3B NRB. If memory serves me correct I heard an early 3B ST after purchasing a new 3B NRB in 94 and only perceived a small difference, not enough to justify the upgrade path Bryston offered at that time.
But my current 3B ST THX is an altogether different beast, the audio spectrum seems extended in both directions with a lot more ambience and detail while remaining very smooth This unit is very good, prior to receiving the 3B ST THX I was looking into replacing my current audio system Celestion SL6S+s), Teac VRDS20, Bryston .5, Rega Planar/Blue Point, but the new 3B ST has made my system (music) sound better than it ever has before, even compared to current systems I have auditioned.
Very often Bryston quietly implement revisions on current units, this ST has later input (Rev 5.3)and output (Rev 5.2)boards and an updated Main Power Board (PWR-MB6), In other words a later ST have a number of revisions over an early ST (better input/output boards) even the top plate is different.
So to answer Krazeeyks question Bryston made a few changes on the ST to NRB which are documented on many sites. What one does not read about are revisions within a current line up. An early ST sounded better than the NRB but a latter ST sounds a lot better if not completely different, probably closer to the current 3B SST.
Hope this helps