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.
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.