A filter should not degrade the signal, assuming the cable company is providing you with a signal that is up to snuff. I've run into trouble with the cable company not being able to maintain a strong enough signal to my house to satisfy digital cable boxes and the cable modem, but a service call or four has led to them putting a signal booster in. In that case, the presense or absence of a isolator was irrelevent, but if I had been using one it would probably have compounded the problem. Bottom line: if an isolator negatively effects your signal, I would recommend getting the cable company out to fix your wiring - either by properly grounding the system to eliminate the need for the isolator, or by fixing whatever is giving you a weak signal to start with.