If you tell them that the problem occurs only when the cable is connected to your system and does not happen when you physically remove the cable TV from the system, then they really have to come out and look at it. We're talking about grounding here, and even as crummy as cable TV service can be this is not something they can blow off - after all, if they're doing it wrong, they are technically at risk of creating code violations.

I can't say what might be the difference in the two that has created the greater sensitivity in the 7500, but it's worth keeping in mind that the two amps are significantly different designs.