You can swap preamp tubes around without having to adjust anything. I'm not familiar with Maeshalls at all, but if you 'borrow' a tube from one channel to swap for a tube in the other channel, you can find out if the problem moves around with your tube-swapping.

If nothing changes, that tends to indicate the power tubes, which are more likely to wear out, actually. Preamp tubes should last a long time, but power tubes take a beating, especially if you play the amp cranked.