The smoothest amp you've heard?

The quality of smoothness is a lack of higher ordered harmonics; also a lack of IMD.

I agree with this as far as it goes.
The whole ariness things needs more definition INSHO to even know if we're discussing the same thing.

@mikelavigne just familiarized myself with darTZEEL as far as reading and that's as close as I will probably ever get! The NHB-468 clocks in at 195k. Maybe I'll find one used on audiomart! I bet though it's fantastic.

Yes DarTZeel is the bomb, but an expensive one. So organic- just music. I have the LHC-208 integrated. Took a long time to work up to getting it. Speakers are PMC Fact 12’s.

You're right it doesn't really make much sense your speakers and room will have 10 times more to do with what you hear than an amp as long as it's used within its limits.

50kHz and 100kHz along with a SuperTweeter to reproduce ultra sonic frequencies is certainly worth considering.

Exactly how or why they would introduce phase shift down to 1/10th the cutoff frequency isn't revealed or explained.

In any case, why just those frequencies? What about 20kHz cutoff frequency causing a 2kHz phase shift?