- A FT-240-43 toroidal core, used to make a common-mode choke for feedlines. Wrap about 10 turns around the core: https://www.amidoncorp.com/skus/search?query=FT-240-43.
- Having some snap-on cores might be helpful - although they aren't as effective as toroidal cores, they're really easy to install, so having a few of them might help identify which cable is the problem, and you could use a toroidal core then.
- A high pass filter for 75-ohm CATV coax: http://www.mfjenterprises.com/Product.php?productid=MFJ-711B
- A ferrite rod.
- A low-pass filter on my station wouldn't hurt.
- What about pickup from speaker lines? In the old days, you'd slap a capacitor across the speaker lines, but my ARRL RFI book suggests that modern audio amps won't like that.