SET Distortion at higher volumes?

@dhcod glad to hear your found a problem and it wasn't a head scratcher.  your amp distorting with 96dB speakers would have been, assuming non-concert volumes.

SET amps are most linear when they are operated at 25% of their rated power: ie 2 watts for an 8 watt 300B. Distortion rises rapidly after that. I have three: a 45, a 2A3 and a 6BG6. They await a suitable speaker after I sold my Heresy's.

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.