Actually Mark, John's suggestion has more merit then just lifting the entire stack from ground.
I do it a little different but the basic idea is the same.
I have found that using a 56K to 68K resistor instead of the stock 6K8 resistor to ground but then, using a spst switch, shunt around the 56k to 68K resistor with something like an 7K5 to 10K (or so), sounds pretty good but gives the amp very serious kick when you throw the switch open. 
When the switch is closed, the two resistors are in parallel and the total resistance to ground is very similar to the stock 6K8 + or - 10%.
Using a smaller values, like 27K instead of the 56K to 68K, keeps the tone controls a little more active and there is less of a boosted sound.
I don't really like the way these amps sound when you lift the entire stack unless you use a dpdt switch and use the other side of the switch to shunt signal to ground ahead of the volume control.
Even then it's dying for some of the tone controls.
I have a simple BF to tweed switching mod I do to BF Champs and such that requires no holes but you have to replace the volume control with a push pull on-off dpdt switching 1m audio pot.

Bruce