ST are the initials of the designer. These are the later model. Non-ST are earlier designed/made. I think they're AB, but surely Bryston has a website, or someone who owns one will tell you soon.
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ST - is for Stuart Taylor, whom I believe is their chief design engineer. Bryston in the past, say from 5-6 years ago (pre ST) to 25 years ago was best known for producing gear suited for commercial applications first and foremost. They also produced nice amps for the home but these amps were not known for their sonics but for superb reliability and service. Subtle sonic nuances were not a high priority until Bryston had Stuart Taylor design the new series. The "ST" versions are amplifiers built to stand the test of time but also provide great sonics. This version would be better suited as a main amplifier whereas the older versions (NRB and prior) are great amps but lack the sonic qualities the ST ones have. If you were wanting an amplifier for build quality, warranty and nice sonics Bryston probably has a ST model for you. If your intent is to say just power a subwoofer then an earlier version (NRB)would probably do the trick nicely. The ST's run with the likes of the Aragons, Classes, McCormacks, etc. ( I feel the oncoming heat of the owners of these products, flame away as I am a die-hard Bryston fan...)
If you need more info e-mail James Tanner at Bryston - they are great people to deal with.
My 4B-ST get really warm to the touch especially after a long period running my 4ohm speakers and not a tear splitting decibels. Music transients, speaker impedences and amp location (in a cabinet) are just a few variables to take into consideration to determine if an amplifier will be quite warm -even class a/b amps