Hiss, hum bad for speakers?

I've got some kind of hiss/hum going on here. Not sure if it is feedback from the high-level connection from my amp to my REL sub or just from connecting multiple amps into wall sockets. The question is, is this hum, hiss, high wine bad for my speakers and gear?
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.
I have a REL Strata III hooked to a Rotel RTC-965 preamp and an Adcom 7400 amp using both high and low level connections. I also have a (quiet) hiss. However mine is actually originating from the preamp. The only reason I say this is because you may be looking in the wrong place. Hope this helps.
I too have a REL sub, but no hiss or hum problems. The REL-supplied high level cable is not shielded, but this should not be a problem since speaker level voltages are going down it, and these should not be susceptible to RFI.

Does the hiss / hum go away when the REL is switched off ? How about when the REL is disconnected ? Do both go away or just the hum ? Is the hiss / hum the same regardless of which source you select ? What if you switch all your equipment off one by one ... does the hum or hiss go away at any point ? How about disconnecting major appliances around the house (maybe even isolate by switching off breakers one by one)? These are the steps I'd go through.

Hum is usually a power supply problem (e.g. earth loop). Hiss : I'm not sure : could be any number of reasons. Some amps are quieter than others, some are more / less susceptible to RFI. Shielding is much more important on interconnects than speaker level cables.

So long as both are well below the normal listening level of your music (as I assume they are or it would be more than a minor problem) I can't see why it would cause any problems with the speakers, but I'm no expert in these matters.
After trying cheater plugs on each of a dozen components and then taking them out of the system one at a time, I've recently changed the interconnects from Nordost SPM to Kimber KS1020. The audible hum is now gone!

It's unshielded vs shielded that eliminated the hum in my system. My preference is the faster SPM but the tradeoff to eliminate the hum was worthy of the change. Change the interconnects.
The speaker cable or ICs might be the cause. I will change them out when I get the chance and see if this helps. Thanks.