I’ve been thinking about this statement a bit. I totally get what you’re saying.
(Incidentally, I don’t know why you’d fear getting “burned at the tonal stake.” Your description sounds like statements many of those here have made for quite some time!)
Anyhoo…let me offer my theory, because it is very much like how I’d describe my experience with my new amp.
When a signal is clipped as it overdrives a gain section, the peaks will be the first to go. The resultant signal is going to sound thicker as there is less difference in amplitude between the fundamental and the other harmonics.
However, when your amplifier naturally produces a more harmonically rich signal (which is still technically “distortion” even though it is not distortion resulting from an overdriven signal) you will still have a thicker sound, even though no overdrive and clipping are occuring.
So, my guess at your being impressed with the Fillmore, even with less distorted signals, may be partially due to the fuller tone inherent in the amp.
Just a theory.