Old thread, may be irrelevant now, but...
I agree with Whoaru, but would say "decent RCA cables", on a 30-ft run, should be at least double-shielded and one of the shields should be continuous foil. Gaps in the normal braid let RFI leak in, like holes in a ship's hull — not an issue with the normal 3-ft run, but very loud when multiplied times 10. I used Mogami's best-quality interconnect cable for 26-ft runs and the hum was intolerable. Switched to cable with a foil shield under the braid and it was silent.
TV / Internet cable companies install miles of cable, highly susceptible to outside interference: such a length is an antenna. They use cable with continuous foil shielding, plus an additional four layers of braided shield around it.
