Problem solved - I've put a DPDT switch in that disconnects the 6V6's (screen grid - PIN 4) from the grid supply (on the board) and connects them to the anode (PIN3) of each tube via a 1K5W resistor each.  Simple mod, and now I have a half volume option.  It rolls off a little treble, but it still sounds great.  It actually has less effect on the amp's presence and dynamics than the triode switch on my Creme Brulee.

Very simple mod that seems to agree with this amp.

Has anyone installed a pentode/triode switch on a Prinzetone?  I like the way it sounds at close to full volume but that's a bit loud for practice/ studio/ late night...  A triode option would be great.  Or any other output power reduction suggestions welcome.