Hum in system when only amp is on

The arrows on the ICs connecting the preamp to the amp should point to the preamp. The preamp AC cord should be grounded, but the amp should not (it's grounded through the ICs and the preamp) and you should use a "cheater" plug on the amp's AC cord plug so that you are only plugging two prongs into the wall, not three.

Try those two things, and see if the hum doesn't go away.

This is not an answer to your question, it's just a rule we ALL gotta follow:
ALWAYS turn your preamp on (and give it a moment to stabilize) before turning on your amp. ALWAYS turn your amp off before your preamp.
Best of luck,
Joe

Nsgarch, I don't think the directionality of the IC's would make a difference in Ian's problem, but my understanding is the arrows should go in the direction of the music -- thus, pre-amp to amp.
I agree that floating the ground on the amps, but not the pre-amp, can solve a ground loop problem. That's what I do, but I've been told it's not the safest solution, either.