Hi Scanido!
I owned a 4B ST and was shocked to discover that even the 9B ST sounded audibly better in my setup.
The new output devices that are cited so often when talking about the changes from ST to SST seem to be only half the story. The 9B ST, being the latest ST design with the old output devices, is very close to the SST in terms of layout/signal path/grounding... The 6B ST and 14B ST are, as James doesn't get tired to point out, besides some small changes in the power supply, identical to the 6B SST/14B SST.
I wouldn't worry about the temperature of the amp, als long as ventilation is good enough. I think it is obvious that Brystons goal is not to manufacture the most energy efficient amps, but the best sounding ones. Besides, in contrast to my 9B ST, my 4B ST didn't really get hot. Maybe you should check on ventilation. The new SST heat sink design might be better in dissipating the heat energy, however, this is of little use if ventilation is poor (e.g. because the amp is in a closed cabinet with no ventilation).
To make a long story short: If you own a 3B ST, 4B ST, 5B ST, 8B ST I would expect a change to a 3B SST or bigger to definitly be "worthwhile".
Since the move from ST to SST brought an increase in power on some models, a move from 4B ST to 3B SST could also be an option if cost is an issue and the speakers are not too power hungry.
Hope this helps
Markus