I just recently built a Ceriatone 18W TMB and briefly had the TMB master volume control installed instead of the PPIMV.  I could not run the TMB preamp volume and TMB master volume above, like you say 9 o'clock, without a lot of distortion.  The problem is the TMB channel has two gain stages while the normal channel one.   So on my 18W TMB I took out the TMB master volume and made the control on the front faceplate a PPIMV (which I dearly love!).   To balance the normal and TMB channel I simply attenuated the tone stack output with a resistor divider feeding the PI.  I used a 7.5 to 1 attenuation which gives the TMB channel slightly more gain that the normal but not much.  I now have a lot of range on the TMB volume control plus before the TMB had a lot of noise   I can also jump channels with ease.

Anyway here is a schematic of my 18W TMB.   Take a look at the treble pot wiper.  There are two resistors: 649K and 10oK ohm to ground.  This is a voltage divider that is attenuating the TMB channel before it hits the PI.   

Scott