This statement ignores just how much loop feedback is needed to fully compensate for this sort of thing. We all know what too much feedback has the reputation to sound like.20 db is plenty. The problem area with feedback is **moderate amounts** between about 4 db up to about 15 or so. Over that and the feedback is usually sufficient to make almost any amplifier act like a voltage source.
IOW George, the quote above was completely misleading. If you doubt me, read Nelson Pass' article on distortion and feedback:
https://www.passlabs.com/press/audio-distortion-and-feedback
Any of the amps that you recommend run 20 to 30 db of feedback! In this case you're trying to play both ends- when all the amps in question run large amounts of feedback.
Consider your bubble popped.