When a tube amp has non-adjustable biasing like on all Mesa amps, they require you to use a certain grade of tubes to insure you don't use a set that's either too cold or too hot.
If you have an amp like this and you buy your tubes directly from Mesa/Boogie, they sell you a set that'll work with whatever model you own and that's enough for most people. If you use Groove Tubes, they use a number system that rates their tubes from 1 to 10. Mesa recommends using a set that's between 4 and 6 if you're going to use Groove Tubes and once again that's enough for most people but not me.
The truth is that Mesa sets the bias on a lot of their amps on the cold side and others on the frozen side. They do this to make the amps idiot proof just in case someone who doesn't know what they're doing accidentally puts in a super hot set of tubes so they don't damage the amp.
Without getting too technical, what I do is use a set of tubes that are hotter than what Mesa recommends because that way, I can get the bias to where it should be and the amp's tone comes alive. The way to increase the bias on a non-adjustable amp is to simply use a hotter set of tubes and being careful to not use too hot a set. That's why I call the guys at Eurotubes. I tell them the amp I have and the tubes I want to use and they tell me if it'll work or not. I love KT88 tubes and wanted to use them in my Mark lll so they sent me a set that would work and wouldn't damage the amp.