2061x goes microphonic with guitar straight into amp

[Papus]

I noticed this when demoing my 2061x into a 1960AHW cab in a store on the weekend.
Couldn't dime every knob like I do at home.
Had to keep all the knobs less than about 8 or it would squeal and fuzz out like a bitch.
Tried it at home and got the same results.
But with a Blackstar HT-Boost in front (bypassed) there's no squeal and I can dime every knob and stand with the guitar right in front of the cab no problem, just smooth fat tone.
What's going on here?

[surfnorthwest]

Has something to do with the input signal it would seem to me. Perhaps with the pedal your getting a higher gain signal to the preamp that is needed to smooth it out. Other then that it beats the shit out of me. I always say if it works go with it any don't question it.