BHK labs measured the 4B3 at 354W into 8 Ohms, and 552W into 4 Ohms. Bryston is conservative with their power ratings. I chose the 4B3 to power my Magnepans and find it to be perfectly neutral and clean. I can not imagine ever needing more power or SPL than is provided by my 4B3. IMHO it is the sweet spot in their line up but I do use Tube Pre-amplification to add texture. There is a difference in input impedance which made the 4B3 a better choice for me. The 4B3 measures at 29k/20K balance/unbalanced.