You're about 75% of the way to understanding. The current issue is both with heater current available, and the high voltage current available for the shunt reg tube. Heater current can be added by adding a filament transformer. Getting more B+ current is nearly impossible without getting a 2nd pair of PT-2's.
The 6CM7 is a fine sounding tube (the small triode is at least), and I would personally say that the SR-45 is the best sounding Bottlehead amp I have heard to date. I should qualify this statement by saying that the SR-45 entails Magnequest Iron from the get-go, and has limited useable power.
