I busted out my Big Muff the other day. I don't normally use it because it sounds like crap in almost every situation. I got it to sound like my bass was taking a sh*t and knew it would be perfect for a doomy mix and started recording. It took me in a droney direction. I think it was key that I had my more vintage voiced/tweeterless old school Mesa cab on hand. The current PH cabs are a little too bright or something for the Muff IMO (I need tweeter for my normal tone).
I had to mess around with my guitar tones a lot... I was dialed in for modern metal and had to change *everything* to get it to work with the Muff bass tone. http://www.youtube.com/watch?v=yhOnebIbXvo
The more I listen to/watch this vid, the more my IQ goes down... just a warning. If you start wondering if it goes anywhere, it goes slower.