Paul,
I fairly sure that if you turn the knob marked 'Solid state & Valve' to solid state, the valve isn't in the circuit.
Worth a try?
Also, as far as compression goes....
Don't turn it past the 12 o'clock position.
(Or turn it off and see if that helps).
Thirdly,
Try it with the EQ set flat, and the EQ balance in the 12 o'clock position.
You could also try turning off the EQ section.
Good luck.