There is no such thing as a cable *modem* amplifier. This is baseband. Accept it. Reducing S/N is the only thing you can do. Why not just unplug the 32 TV sets you already have connected to your home cable service? Or get the cable company to remove the odd staple or two from the cable drop over your roof that might interfere with you favorite dub-dub site?

Either way is is probably just a ground loop attenutaing signal. It is their problem. But you seem to simpy to deal with it. Think DEMARC here. You have a PC, you subscribe to a service, it fails.

Grow balls. Call them. Tell them to FIX MY F'IN' CABLE INTERNET...NOW!

That's what you pay them for. It is likely simple physics and it not your concern.