Hiss, hum bad for speakers?

Are you talking about a clearly audible (loud) hum, or is this something where you have to place your ear close to the speaker to hear? It's a rare system that doesn't produce some of the latter, but the former should be addressed. It's probably not hurting anything, but it certainly won't help your enjoyment of music. I suggest you check the REL connections at the amp and make sure your power cords aren't touching any signal carrying cables.

In my experience, a hum is almost universally associated with a ground loop. Do you have a cable TV or satellite system hooked up to your system? Have you tried to drive your REL from the preamp instead of using the high level connections at the amp?

No cable or satellite hookup, just DVD and CD. I could run the REL from the preamp, but that would activate the bass management and would not be true analog. And yes, the hum/hiss is loud enough to hear from my sitting position. The high level cable from the REL is not really touching any power cords.

How do you connect your sub ? from preamp using RCA terminated cable(s) ? Or from amp using speaker cable(s)?

Either case, replace them with the shielded one.

The REL sub is connected using the supplied high level cable that attaches to both positive terminals on the amp and one ground terminal on the amp. I did this following the REL manual instructions. It is the only way to get proper 2ch music use out of the sub without going through the digital processor. I believe the high level connection is shielded.