I have a EH Big muff pi, and although I really like it im not totally satisfied with it.

I was wondering if anyone had some good ideas on how to tweak it and give it a better sound.

I will post a picture of the circuit board if that will help (disregard the red writing)
If you have the Russian version, you could replace all the reistors and capacitors with higher quality ones.

Install a 9v dc jack so you can use mains instead of just batteries.

True bypass could also be a good idea if you don't have a true bypass looper.
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EHX pedals are really hard to mod for true bypass.

Luckily I just finished my true bypass looper and it works perfectly.
Depending on the sound you want, yeah, you can change out the clipping diodes.
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what kind of diodes would be good to switch in?

Well, it depends what sort of tone you're looking for. LED's will give you a bit less volume but sound more 'crunchy' (best way I can think of describing it). Germanium diodes (1N34a's and the like) will give a bit smoother tone, while normal signal diodes (what you've probably got in there now - generally 1N914's) will be a bit more distorted and not as smooth. You can also use MOSFET's and FET's for the internal diodes, they are meant to sound more tubey, I haven't had much of a play around with them yet.
If you want to smooth the tone up a bit, replace all the ceramic caps with silvered mica. Next work out what components are you tone controls and have a play with different values of capacitors.
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