I'm not sure grounding the amp to the power conditioner will make a difference. Does it have three-prong plug? If so it's very likely that the chassis is already grounded to the power conditioner (and everything else). Nonetheless, if you still want to try it as a
temporary test to see if it makes things better, you could use just about anything. You could use some Romex lying around, a piece of speaker wire, and old extension cord that has the ends cut off, etc. If you don't have any of those things, you could go to Home Depot and get some 12 gauge wire off the big spool. Don't use something with a very thin diameter such as phone line or rRCA cables.
I think something's wrong with one of your pieces of equipment. The fact that it goes away after a while, makes me think it's electronics. If it were me, I'd swap out components until I figure out what it is.
- What if you turn on only the amp, but not the preamp?
- What if you swap out the preamp and then fire up the amp and the preamp?
- What if you swap speakers (I think it will still occur).
Like I said, I'm betting on electronics, so I'd try to figure out if it's amp or preamp. Then go from there. I don't think the grounding exercise will make a difference, but give it a try!
Good luck.