Ah OK, well that's different.
How do you EQ your amp? I used to own a GK, too, and I learned that the sound you hear on stage is very different from the sound out front. Usually, when it sounds nasty and aggressive on stage, it's just about right for the audience. Maybe you could have a band member listen to your sound from a distance during soundcheck.
And of course, bringing your amp to your next lesson is a great idea!
Timo
PS: Merry Christmas, everyone! I turned down all Christmas eve gigs in order to spend the day with my family (and also reading TB

) for the first time in years and I must say that it's great!