Class A Watts vs Class A/B Watts

It's impossible to answer, as there's some missing information. Clearly, a class A/B amp delivering 100 watts into some load is twice as powerful as a Class A amp delivering 50 watts into the SAME load. But you seem to be wondering what happens for other "difficult" loads. And that's where the only possible answer is "it depends"... on the impedance vs frequency of the load, the output impedance vs frequency of the two amps, the gain vs frequency of the amps, and the power spectral distribution of the signal. And if the load causes either amp to go nonlinear, then it really gets messy!

I'm interested in the same question. As a possible example scenario, what would the performance comparison be between a Krell KSA50S (50w / class A) and a Krell KAV-250a (250w / class a/b) driving relatively insensitive speakers such as the Dynaudio Contour 3.0's?

This one is pretty tricky. Some Class A/B amps only run Class A up to a VERY FEW watts. The better ones will do at least 10 to 50 watts, depending on design and construction. Be forewarned though that a "real" Class A or richly biased Class A/B will run HOT. You will need very adequate ventilation at the minimum. 130* to 150* F at the heatsink is not that unusual when idling. Don't try stuffing the amp into something with closed sides or backing, especially if it has side mounted heatsinks like Krells, Thresholds, Perreaux's, etc... As to a low power amp being able to drive a difficult load as well as a high power amp, that has little to do with what class of operation it runs in. A well constructed amp will drive just about ANY load as long as your not expecting to raise the roof all the time. I've used an amp rated at 40 wpc Class A( Forte 1A ) to drive 82-83 db speakers to good sound pressure levels without any problems. While this set-up would never really "jam" at rock concert levels, it worked fine for most "normal" listening. On the other hand, the same speakers used with a MUCH more powerful Class A/B Bryston 4B would occasionally bring the clipping indicators into action. While the Bryston was louder, it never came anywhere near sounding as good as the Forte' did at lower levels. Sean >

Sean makes a lot of good points. A good amp(and we are generally speaking of solid state in this thread) will be able to drive difficult loads. A classic ss amp doubling power as impedence is halved. Usually, I would opt for the class A amp. Remember those Bedini amps of the 80s? They made some of the best solid state sound around. Class A and 25 or 50 watts, they had NO trouble driving most speakers to the desired volume, and it is truly a loss that they're not around any longer.