Loud Hum When Pre-Amp Off

I get a loud hum through speakers when I turn on my ST-70 but my pre-amp is not turned on.
When the pre-amp turns on it goes away and there is no hum. Does this mean I have voltage leaking into the chassis or something
that the pre-amp is grounding out when it gets turned on? Touching the amp during humming seems to change hum and there maybe even is a little shock, very little.
If this is the problem, where should I look? The wiring all seems kosher to me, so I'm not sure the most likely places to look.
This is one of Bob's kits by the way.

In the professional audio world, it is standard practice for power amps to be powered up last and powered down first. This is primarily to prevent any pulses caused by switching transients from damaging the loudspeakers. I have also found this to be a good practice in my home audio system. I believe if you follow this sequence you will solve your hum problem.

Best Regards,
Charlie

Thanks for the input.
I always turn on pre-amp first now.
I'm just concerned that there may be something wrong with my wiring in the amp.
Sounds like that probably isn't the case, but rather the hum is related to my pre-amp's own grounding scheme.
There certainly is no hum or other problems when the pre is on.
If I misunderstand feel free to enlighten me further.

Disconnect the RCAs from the AMP... Does it hum when on?

mantha3 wrote:Disconnect the RCAs from the AMP... Does it hum when on?

I discourage you from doing this without first shorting your inputs using either shorting plugs or a short single RCA cable. The cable should be as short as possible so it doesn't act as an antenna and pick up RF and transmit it into your amp. If the inputs aren't shorted, your amp could oscillate and potentially damage your speakers (and make some horrible noise!). It is absolutely the correct procedure to power up your pre before powering your amp up. Always.