The Mark IIA and IIB series of Mesa Boogie amps contain a strange but fortuitous function, available only on these early Mark amps, since the IIC killed it off when they implemented a very nice effects loop.
I believe the IIB had a tube buffered effects loop as well, but the IIA had none. But what both of these amps did have was a junction jack. It let you take the signal out, right after the pre-amp and right before the power amp. On my Mark IIA, it’s the jack circled right here:
This preamped signal contains everything the Mesa has to offer short of the power section. That’s right, even the Master Volumes for the Clean and Lead gain stages, the boost, the treble on, you name it, it’s before it. What do you do with this preamped signal? Plug it into a volume pedal. Then out of the volume pedal, you plug back into the amp over here:
This way, you have one last volume control before the power section sends your signal to the speaker. In other words, you have the ultimate Master Control, the master of all other master controls. In this way, you can dial in both channels’ volumes and gains, their master controls, and you’ll still have one more volume control after that to tailor to taste. Makes for some super cool volume swells too, done after the preamp instead of having to be before the amp, and thus you can even swell your overdrive and distortion signals without losing any gain. COOL. Here’s an example of my amp using this trick for both channels (technically they aren’t “channels” but heck, i set them up to be Clean and Dirty so I call them my channels).