Older Bryston 4B vs 4B ST

I'm a newbie looking for an amp to go with my Aerial Model 6 speakers. I'm currently using a Krell 300i integrated. I auditioned the new Bryston 4B SST and really like the sound - however, I don't want to spend $3100 on an amp and was looking at the used Bryston 4B/4B ST amps here.

My questions is, what's the difference between the 4B and the 4B ST. Is it a small difference in sound that is only a matter of opinion? Or was it a serious upgrade?
I own the 4B-ST and have auditioned the 4B-SST several times. Frankly, I can't hear much difference -- we're talking subtle differences at best. The 4B-SST has a bit more power than the 4B-ST, but the power increase won't be audible except to the extent the 4B-SST has a bit more headroom to handle dynamics. The other sonic difference (to my ear) is that the 4B-SST is a tiny bit more transparent and seems to "disappear" a bit better than the 4B-ST. Again, however, I want to emphasize that the differences between the two units are subtle, and only become apparent upon extended listening. Your Aerial speakers are good enough to reveal improvements to your amplification, but my honest recommendation is to buy a used 4B-ST (typically around $1400), and use the rest of your budget to upgrade other parts of your system.

The owners of new 4B-SST's may have opinions that differ from mine. I really like my Bryston 4B-ST, but if I were going to spend $3100 on a new power amp, I would probably upgrade to something like a used Levinson, a Ayre V5x, or a Theta Dreadnaught.
I think it might depend on how old you're talking. I have a fifteen[roughly] year old 4B-THX[NRB?] that sounded very bright when I tried it in my two channel rig. But I bought it only intending to use it as a sub amp so musicality didn't really matter.

There has been discussions in the forums that with every upgrade the Brystons became more musical. I'm just guessing, but the difference between an old 4B like mine and a newer 4B-ST might be big while the 4B-ST and SST might have a closer sonic signature.
I concurr with Gunbei. I have owned all 4B flavors other than the new 4BSST. The major difference between 4BST and order 4NRB is the noise level due to the improve signal path design on ST version. The input signal path on 4BST is a lot shorter and much quieter. Also, if you are looking into the 4BST, make certain they are the later model with new style power transformers. The older 4BST had mechanical hum from noisy power transformers. Same goes for 7BSTs.