SET Distortion at higher volumes?

Depending on what you have many lower priced  amps have sheet metal cases that ring ,vs say a Ayon  brand which is machined aluminum .

your tubes need dampers Herbies tube dampers on All tubes especially power tubes vibrate and ring , if you tap your chassis and can hear it through your speakers you have bad microphonic issues start with the tube dampers. 

Spatial Audio Labs make a very nice open baffle that's very efficient 

and has powered Bass . Which work great with SET amps.

using the excellent Beyma midrange tweeter amp driver + Bass .

Tube dampers dull the sound for me.

The problem was traced to the tonearm. The amplifier is not the culprit.

Charles

We can tolerate lots of even order tube generated harmonic distortion before sound becomes obviously distorted. Lots of digital pop and rock recordings are mastered using tube compressors or other tube signal processors that generat harmonic distortion in to make them sound fatter and more "analogue".