its a little late but this info fits your question exactly but not totally:
the differences between the NRB and the first ST are partly available in the NRB when using the balanced inputs. The exact explaination is available in the Bryston published "critical acclaim" in the amp section [vol 5 page 86] the article from[and obviously available from] October 1996 vol.19 no.10 "Stereophile" in the outake titled "the Bryston ST's" quote "it was evident to Taylor that the NRB series' balanced inputs sounded better than the unbalanced..." this difference Bryston calls their proprietary input buffer, I don't know what it is but its in the NRB,s balanced inputs only, whereas in the ST all inputs have it. BTW, the NRB can be factory upgraded to full ST , but not unfortunately to SST [however i think a ST can be upgraded to a SST] A very interesting subject that was mentioned above remains beyond my ability to obtain, that is a cronology of the upgrades/changes the Bryston line has received, in the case of the 2B 3B and 4B, since 1977. i shall post this very question.
the differences between the NRB and the first ST are partly available in the NRB when using the balanced inputs. The exact explaination is available in the Bryston published "critical acclaim" in the amp section [vol 5 page 86] the article from[and obviously available from] October 1996 vol.19 no.10 "Stereophile" in the outake titled "the Bryston ST's" quote "it was evident to Taylor that the NRB series' balanced inputs sounded better than the unbalanced..." this difference Bryston calls their proprietary input buffer, I don't know what it is but its in the NRB,s balanced inputs only, whereas in the ST all inputs have it. BTW, the NRB can be factory upgraded to full ST , but not unfortunately to SST [however i think a ST can be upgraded to a SST] A very interesting subject that was mentioned above remains beyond my ability to obtain, that is a cronology of the upgrades/changes the Bryston line has received, in the case of the 2B 3B and 4B, since 1977. i shall post this very question.