Hi Andrea and welcome to the forum.
I'm not familiar with your particular amp, but from a quick look at the top panel it seems you're getting a volume mismatch when switching channels (actually there's a bit more to it than that, but perhaps another time...). Unfortunately it doesn't look like there's a solution just using the amp. The issue is that everything goes through the same gain control. It's done to keep costs down, but the downside is that you sacrifice flexibility. More expensive amps tend to use separate circuits for the different channels. The upside is that you have control of settings for each channel; the downside is that they're, well, more expensive.
The obvious thing to do (notwithstanding what you've said) would be to get a pedal. All overdrive/distortion pedals come with their own output level control, so you can just leave the amp on the clean setting and adjust the output level on the pedal to match it. (You probably know this already but just so I've said it, the pedal will have a bypass switch as well so you can send the signal straight through to the amp for clean sounds.)
There's literally dozens of pedals to choose from whatever style you play, and the good news is that you don't need to spend a lot to get a good one.
Hope this helps.