Don't know if you saw this thread, but it really gets to the nitty gritty regarding the Master Volume Boost:
http://mtsforum.grailtone.com/viewtopic.php?t=2561
After playing around with the MVB on my RM50B for over a year, I've come to the conclusion that it is a pointless, close-to-useless "feature". Keep in mind I love my amp, but the MVB just doesn't make much sense IMO.
As Rich mentioned, there are better alternatives. I've been using a Dano Fish & Chips EQ in the serial loop for quite some time....much more versatile and works like a charm. Running a volume pedal in the serial loop would be another alternative if you want on-the-fly variable volume.
BTW, awhile back somebody with more amp tech knowledge than myself pointed out that running the master high and preamp low doesn't "cook" the power tubes. It does sound different, but it's not because the tubes are cooking.