For a better picture, my rig:
AMP: Koch Studiotone 20
GUITAR: Ibanez AS93.
Two ways I use the Muff: lead solos or single-note lines on the lower strings (riffs). I run it into the clean channel of my amp. I tried using it into the drive channels but they have a lot less headroom and the pedal chokes out in lower frequencies. This shouldn't be a problem on higher headroom amps though.
I found unity gain to be somewhere around 9 o'clock. Plenty of volume in reserve - this is one loud pedal! Tone knob takes you all the way from woolly color (while still retaining clarity!) all through rounded singing lead sounds into nasty buzzing bees territory. It has several sweet spots along the way, where the sound just suddenly pops forward. Sustain knob all the way down still overdrives the sound quite a bit and sustains literally forever when dimed (go figure).
The pedal does scoop some mids but not as much as the other Muffs. It is, therefore, best to use into an amp with a slight mid hump (sorry, Fender). Also, it does have a slightly metallic sound to it which I can't really put my finger on. But I quite like it. With the amount of drive the pedal puts out the dynamic range in terms of picking is not too wide and The Muff is not a pedal that cleans up at all by lowering the guitar volume. It is the kind of pedal you just turn on and get THE sound and that is it. Which is fine. It does add a LOT of background noise when you turn it on, so be warned. Not an issue while playing, though.
The build is great. Chassis is sturdy enough, knobs are firm, the switch is really stiff, you really have to press it down and it makes a loud (also tactile) click when engaged. Personally, I'd prefer a softer switch and no click, but it's ok. Led is not too bright and clearly visible. I had to peel off the rubber legs in order to put pedal-board tape on it, though.
Overall: 9/10 would buy again.