i have had the fw f5 and f6 as well as the pass models i mentioned earlier (currently have the int150, xa30.5 and a modded f5 in my collection of amps)
nelson pass has been interviewed many times, these vids are on youtube, he speaks at length to steve guttenberg and others about how each gen of pass amps, different generations of them, and different fw amps are all his playing with available components (especially various transistors, some no longer available) in various ’less is more’ circuits to produce amps of different capability levels and with varying sonic traits... he is very articulate about how different types of distortion leads us to hear certain attributes in the music
there is also a terrific thread on one of the other forums where he directly participates, and he speaks about his take on how various first watt amps sound different within their respective power envelopes...
these interviews are worth watching, easily found on youtube with a basic search, as is that excellent discussion thread where you hear the sound of various of his described by the man himself