Loud background noise: cables picking up RF?

When is the noise present? Always ? With or without a signal present ? For all sources ? etc. Does the volume of the noise change with a change in the position of the volume control? The first thing I would do is find out what the noise looks like and what frequencies are present - audible and inaudible. Scope the various points in the signal chain to see where it is present and to see what it looks like. If you don't have a scope - buy one - Kenwood sells fairly inexpensive scopes new and you can probably pick up an old tecktronix cheap. Its not like you'll never have another use for it. You are not hearing transmitted RF through your speakers - you cannot hear RF and your speakers cannot reproduce it - outside the frequency range for both. By seeing what it looks like and where it is you can do more than guess at it.

"I get the noise as soon as my preamp and amp are engaged, whether or not I have a source hooked up. Can I buy a cheap AV-style IC from Best Buy and and see if the cables are the problem, or are those low-end IC's they sell really shielded? Are the power cords also a likely culprit?"

In the previous post I did not address this part of your post. The power cords are not the problem. The wire feeding your outlets will still be there and that is not shielded. RFI filters immediately inside your preamp and amp should take care of that anyway unless it is of extremely large magnitude. In which case you may need an external RFI filter. Again - using an oscilloscope to troubleshoot will make life much easier. As to ic's picking it up - maybe. Any shielded IC should work fine. What is important is that the shield is complete and terminated correctly. Make a pair of IC's yourself, terminate the cable so that only one end of the shield will be connected to ground when it is hooked up - make that end the source end in each case. Radio shack best buy or whatever - no need to go to an expensive cable. To minimize any problems as to coverage of the shield (if you are using a mesh vs foil shield)- avoid sharp kinks in the cable. As a quick check- try adjusting the position of the cables that you do have hooked up now. The effectiveness of an antenna for a given signal depends on its orientation - if you start moving the ic's while listening - as if adjusting an antenna - and you hear a change - then yes, your ic's are definitely picking up the interference.

No Jea48 you are incorrect - his speaker wire will not act as an antenna that causes transmitted RF to be heard through his speaker - that makes absolutely no sense - if that were the case he would be hearing RF all the time, with or without his amplifier on or connected. In every system on this planet the speaker wire is past the amplifier not before it.

Jea48: After looking through my own copy of ARRL's Radio Frequency Interference: How to Find it and Fix it, (sec. 2-8, 22-11, and 3-18) and looking at the schematic of a couple power amps I tend to agree that it is possible to inject RFI via speaker leads. What was troubling me was the mode of getting it through the amplifier. It appears that this is possible via the feedback path all the way to just past the differential stage. Having been a Ham all my life I cannot say that I ever had any complaints from my neighbors or family when I was transmitting but it certainly appears that it is possible to feed the RF back through the amp in this manner. I would think however, that the active components would end up detecting the modulating signal and you would end up with voice at the speakers, at least in the Ham setup (unless of course transmitting CW).