What the Pass XA.5 series does to Class A signal power when speaker impedance is reduced from 8 ohm to 4 ohm is well discussed on this board. Secondly, the design differences between Class A and Class AB amplifiers can also be searched through this board. A good number of contributors to this particular message thread is aware of these facts. Furthermore I own the XA60.5, and I talked to Pass Labs personnel at time. When impedance is halved, power doubles for Class A signal. And that's just the way it is for this design.
Pass Labs A/B amps vs Class A amp and Actual Power
Dumb question; how can you compare the actual power or ability of an amp to drive a tough load (such as a Magnepan) when looking at a Pass Labs A/B design such as the X250.5 or X350.5 vs one of the full class A amps like the XA60.5 and XA100.5? For example, which amps would (from a standpoint of just pure power/grunt) drive 3.6's better, an X350.5 or a XA60.5?