Let's hope I can make myself clear
I have the Vox AC30 and use Radial Bigshot so switch between the two channels.
I only use top boost for high gain distortion, i.e. high channel volume. As for the Normal channel, i prefer the tone to remain clean, not breaking up into distortion, i.e. low channel volume.

This is problematic since the volume difference between the two becomes immense. In addition, distortion sounds awful on low master volume.

How can I solve this? Are there any useful equipment?
I dont think AC30's are hi-gain amps, which is part of your problem. Your not using it for what its best suited for, and its a really single channel amp with a second mode/top boost, which is boosting the amp, but since the volumes are all intergrated into the same channel you get bigger volume differences than you would with a true 2 channle amp.

Amps like yours just get dirty as you raise the volume on it what ever mode its in, just the nature of the beast.

You probabley want to get somekind of overdrive/distirtion pedal for it.

I think Vox has lied a little with marketing, since amps now-a-days are rated by features more than tone by more and more people.

Nice amp though, wish I had one!