I just went out on a limb and picked up an ElectroHarmonix Metal Muff...
..I think I found my new "main" sound!
The trick I've found to using this pedal is to keep the mids nice and high (no big secret there), but also to engage the "top boost" function, and use the boost EQ knob to balance between the hyper-scooped non-boosted sound, and the super-nasal sound with the top boost wide open.
I've been happily playing on it since I got it, and no signs of slowing down. You know you've got a winner when it makes you want to play more! So here's a more in-depth review:
First, the design. It's a metal box with latching-style footswitches. The only things that could be damaged are the knobs, and that's an easy enough fix. Of course, I keep my pedals on a rack tray and switch them with a GCX, so no worries of it breaking.
It has 2 "flavors" of sound: the raw, standard Metal Muff sound, and the Top Boost sound. The raw sound is just that: very scooped, but with enough control in the EQ section to bring it up to reasonable levels. I never turn the mids lower than half on mine, and it's plenty heavy. It has a strange cocked wah kind of sound...a subtle "aw" where the midrange is scooped out in just the right (or wrong, if you don't like it) place. That's where the top boost comes in. It was *originally* designed to place the emphasis on a different section of the EQ spectrum, slightly higher in the midrange than the non-boosted pedal sound. This is allegedly to make it cut through a mix. I actually use it as I said above: to balance the two sounds, using the top boost EQ control as a "mix" knob.
It is also easy to chain with other pedals. I tried it with a boss CS-3 compressor, a Keeley TS-9 Plus, a TC Electronic Nova Drive (using the overdrive section), and hopefully soon I'll hit it with a BK Butler Tube Driver. It takes it all very well.
My only real complaint with this pedal, is there's no external switching for the Top Boost function. It would be easy enough to perform this mod myself, but I pretty much never switch it off anyway.