First rule of Mesa cleans: set the master at least three-quarters of the way up. You really, really gotta crank them, since you are compensating for the loss of gain (which means volume, too) that happens in the clean mode. That channel has a crunch mode that adds a gain stage, so the gain and volume knobs need to cover the full range of all three modes. You're running on the first mode, where there is only one gain stage, so just crank the channel master and keep turning the gain up until it starts to break up, then back it off.
After you do this, you can set the other channels to match. Keep in mind that matching means the channel masters will probably be somewhere around 10:00. You're not playing a modeler, which means the cleans are going to be much quieter than the gain channels, since the amp uses tube gain stages, and not compression, to create saturation. Don't compare it to Engl or Fryette, either, since those amps work completely differently from every tube amp I have ever played.