I know It sounds better in first place of the chain...But the problem is that when I roll back the fuzz level, the sound became darker and quite unusable. I've try this with the MJM London Fuzz and the Analogman Sunface with the same result. When I try to put a pedal in front of the FF, the fuzz level was operating better...But the sound was getting crappy!...
Any idee to resolve this problem?

Maybe try a clean boost or some other type of buffer in front of it.

Most fuzz pedals sound best with the "fuzz" control turned all or most way up. What's cool about fuzz pedals (especialy the Fuzz Face) is the ability to control the amount of fuzz with the guitars volume knob. When you turn down your guitar with a good Fuzz Face, the sound cleans up, and actually has a nice clear bright sound to it. I've gotten better cleans with my fuzz on then when off!!!

If you put any sort of buffer, or non true bypass pedal in front of a fuzz, you lose the whole clean up thing. It's best to have the fuzz the closest to the start of the chain as possible.

Head