I have a new DSL 20 head and when I use the loop, as soon as I activate a pedal, I can hear that pedal coloring the sound. Once I switch the loop on via foot switch, the volume drops a bit, but I get the same sound. Once I turn the loop back off, the coloring ends and the loop acts as it was intended. But there is a volume drop.
Please can we ask what pedals you are using in the loop?