A muff is fine if you roll back the sustain and turn the tone knob forward, but then you lose(what I find to be) the 'sound' of the Muff.
The Rat is probably your best bet. Really solid little pedal, and even though there are a variety of them from over the years, you won't be disappointed by any of the standard Rat pedals. I do find I prefer my 80's Rat compared to my more modern one, but if you can get one for $60 instead of close to triple that, go for it! If you can find one without an LED, hop on it, and I don't think you'll be disappointed.
How are you finding the Warp? I had one very briefly, and found the clean channel to be fantastic, but the distortion was a little too bland. Not a terrible head, for what it is, though.