"Prolly phasing each other"? What does that mean?Dude the two tones are prolly phasing each other.. Instead of working together.. Creating a dull tone.. I didn't like fenders new rumbles when I tried it.. Sounds off.. Try pluging the mxr pre into the return loop before the power amp.. And see how the pre sounds.. If you don't like that.. Return the mxr.. And get a mxr micro amp.. To boost the fender.. Instead of trying to flavor the amp with pre.. Now if the fender by itself self sucks.. Return it and try a galen kruger or harkte.. Or just make an amp board..
Anyway. I have an MXR M81 and a Fender Rumble. The M81 is a great preamp and has never made any amp I have plugged it into sound worse. I don't care for the sound of the Rumble. Sounds like a different amp would be the solution.