I think George was referring to me when he said a member here who changes the Mosfets for Bi-polar transistors. I agree with George on what he is hearing in sound differences between the designs. This came about as I repair Counterpoint power amps which used mosfets and then Mike Elliott redesigned the amps because he could not find the Mosfet replacements and then changed the design switching to bi-polar transistors. For the Counterpoint amps you can almost swap out the mosfets and install bi-polar transistors with a few changes and bias adjustments. Since we repair so many of the amps, I can compare the sound to one another. We now build our own amps after the new design that we implemented when we completely rebuild the Counterpoint power amps point-to-point wired basically using the chassis and the transformer so they are no longer Counterpoint amplifiers.
You really do have to have a trained ear to hear the differences especially when you compare them side-by-side.
Happy Listening.