Ah, the good 'ol Hunt for The Hiss... BigE is right on the money with some basic trouble shooting. There are a lot of things you can try to eliminate your humming woes. BTW if you are playing a single coil equiped guitar that doesn't have proper pickup shielding or reverse wound middle pickups you can get a lot of the dreaded 60 cycle hum.
1) GROUNDING. The most important factor in eliminating hissing and humming. I like to keep my amps and pedals seperated from the same power source. A Voodoo Lab Pedal Power 2 is HIGHLY recommended to power your pedals quietly and efficiently. The amps are powered on their own specific power strip. I also use the VoodooLab Amp Selector (I use more than one amp) which helps me isolate each amp from each other which really cuts the racket down. Other than that, high quality cables help immensely.
On a side note, sometimes you'll always have some kind of problems with noise at certain venues. As Big E mentioned, some older houses or buildings have very dirty power or tons of neon lights. Those are a nightmare for your rig lol. Perhaps investing in a nice power supply/conditioner like a Furman would help? Good luck stamping out that noise!
'67 and '74 Fender Twin Reverbs, '74 Marshall 1987 lead mkII, Metro Superlead 100. Pedals from TC Electronic, Ibanez, Dunlop, BK Butler, Electro-Harmonix, Fulltone, Maestro/Gibson, Loopmaster switching, VoodooLab, Boss. Gibson and Fender guitars, Dimarzio pickups.