Hi guys, I recently ran into some issues with the power section of my MKI 212.
The tubes had been changed less then 10 playing hours before this issue occured.
My master volume was all over the place, any slightest touch of the potmeter would blow the volume in any direction. Even with the channel volume turned almost down, the (master)volume would fluctuate from dead silent to earbleeding loud with hardly moving it.
So, I brought the unit to an ampdoctor for repairs, turned out the masterpot was physically worn out...because of the channel volume pot.
He explained to me what has been explained in this thread already; channel volume should be low at all times, so that the masterpot can be turned way up.
The thing is, the channel volume is a 'digital' setting, it's part of the moddeling infrastructure. The masterpot however gets an actual load to bear, the more it's turned down, the more tubepower/current it has to 'block'. So turning the masterpot down will wear it out sooner.
It was an expensive repair, the tech had to build a new potmeter by hand (no official Line6 replacement part available in the Netherlands unfortunatly).
Not sure if this direclty relates to OP's issue, (probably irrelevant since it's been 3 years) but maybe my case helps on other mastervolume-related issues :)