The only Muff type I've ever really liked on bass was the ThorpyFX Fallout Cloud (Used to be called the Muffroom Cloud until EHX got all lawyery)
It has an active 2 band EQ and it makes such a difference. Ideally it would be a 3 band, but a good mid hump can be set by reducing the Treble and Bass leaving the middle where it is and then upping the output to compensate.
It's a great pedal. I'd probably go as far as saying, if it was 3 band and had a clean blend it might be a contender for the best muff type ever built.