what is your amp, again?
You might not be able to do too much to get rid of ALL the hum, but the signal from your guitar should swamp it.
which volume do you have off? if the guitar volume is off, and the tuner pedal is on, and your noise gate is in front of the amp (between guitar and amp input) then the head will still be making noise and passing that to the speakers. Crackles can be from valves and unless they are really frequent, loud and noticeable I don't think that tiny crackles are abnormal... maybe someones knows a bit better than me though.
You might have some 50hz hum, and its possible to get 100hz hum (in europe at least). That is 50hz from the mains electricity frequency and 100hz being a harmonic of that. most amps have a little hum. If you think it is really severe take the amp to a tech.
You do mean hum and not hisssss right?