Hi. when I connect my guitar directly to the firebox it's sounds fine...

but when I kick in my tonbone distortion, it's sounds horrible.
super-overdrived, compressed. as if it can't handle the signal....

what can I do?
I'm guessing that the signal is too strong for the input. Have you tried trimming it a bit?
It's probably an impedance problem. Turning the output volume right down on the pedal should help.