Here's your issue: when you put the VH4 pedal in front of an amp (clean channel) then you have the sound coming from your pedal, wich is then colored by the preamp, and then sent to your power amp. in a serial effects loop, you are placing your pedal inbetween the clean preamp of the amp, and the power amp. so you are giving your pedal a boosted signal. why it is fine one one amp and not the other depends on your clean preamp settings. it's much better that you use the preamp out on the VH4 pedal, so guitar straight in the pedal and then connect the pedal with the preamp output (wich is the boosted output of the pedal) right into the FX return of your amp, so you are not using your amp's preamp. Glad to help
