weird...it should reduce it.. 
Yea, I tried several cap types last night, same issue. To be clear, I'd expect a good amount of hiss for this much gain but its unbearable and beyond what I'd expect and more than I could stand for a live gig so I have to do something to calm it. I didn't notice a huge sonic difference by removing that cap (other than the hiss being much reduced) other than when I used larger and larger values it started shaving off high end. Not knowing it's true purpose I'm assuming it needs to be there (oh wait you said should reduce it)? Without that cap, the hiss that remains is exactly what I would expect with that type circuit.
Just to add, this unexpected hiss gets worse as either the gain or guitar pot hits zero. It's bad already but once it goes to zero, there is that little extra few dB of hiss that I noticed. I may record it if anyone is interested. I want to build this baby, but need to make sure I understand this piece first.
