Hi Folks,
Sorry i have not commented earlier - sometimes I feel I stick my nose and my bias's in to these discussion way to early.

Anyway
NRB - terrific Bass slam and great Transients - a little forward sounding in the mids and tweets
ST - Smoother on top with a little less slam in the bottom
SST - refinement of the whole spectrum better control in the bass and better at mids and highs in balance with the bass and also soundstage placement improvements.
SST2 - different across the board. Similar sound at low and loud levels. All models can drive lower impedances with more current delivery. Excellent balance of sound spectrum and better control of really deep bass and soundstage placement does not vary with level. Also the noise spectrum is approaching theoretical limits. New power supply design provides terrific transient capability

So if I was buying used the SST2 is the better unit if your looking at overall musicallity and drive capability with most real world speakers but the other older models may fit a specfic speaker or taste. For instance a subwoofer using the NRB would be excellent

james