James;
The answer depends on whether the less-than-3-Ohms being contemplated is over a narrow band, (which is perfectly fine), or average over the full frequency range, (which could cause overheating).
The 7B SST and the 28B SST are both Series amplifiers, and either can drive 3 Ohms and above without difficulty, so they are happy with low impedances. The 28B test showed it to have more THD at 2 Ohms than at 4 or 8, but still extremely low distortion, (under 0.01% full-band) Show me another amplifier whose THD at high power levels is below 0.01% at 20KHz and above even at 8 Ohms!
If you are planning on a speaker or speaker combination whose overall impedance is well below 3 Ohms, we recommend contacting the factory for more detailed analysis of heating concerns, etc.
cwr
Bryston Engineering