Hi,
It's fairly rare for RCA cables to be inherently faulty, but you can check each one by first, setting the volume control on your preamp to a moderate level, then selecting each source you have connected. Grasp the cable where it enters the RCA plug at each end and gently wiggle it back and forth. If you suddenly hear a big increase in hum, then that RCA plug is likely intermittent or defective.
Yes, it's normal to get a sudden increase in hum if you touch the hot terminal (the center one on a male RCA plug) when the other end of the RCA cable is connected to a sub or an amplifier or your preamp.
So is this a stereo setup or do you have any video source, including a TV monitor, connected in any way to the system?
One more thing: does the preamp have a headphone jack? If it does, then disconnect all your sources from the preamp, set the volume mid-way through the range of the control and plug in a set of headphones. Does the preamp by itself have audible hum? If so, then get it repaired under warranty.
If there is no hum (it should be silent), then one by one, connect each source, turn the source on, and listen to each one. If there is suddenly audible hum, then you know that source is the culprit. So try running that source from a different AC outlet or reverse its plug if possible.
I'd also remove the line conditioner totally from your system and connect each source individually to the wall outlet while you listen for hum.
