Thanks guys for a lot of good advices!
I've done a tone-test (git straight into the amp, in the loop only the vol-pedal). If the vol-pedal is in the loop you hear a degredation óf the tone ... loop disabled and the amp "shines". Not tried to use the vol-pedal infront ... I guess that would also effect the tone.
I'll try some active vol-pedals to see if that helps.
The best way to control the overall-volume would be between the amp and the cab ... but I'm not aware of anything that could do that (without harming the amp...)