CATV grounding can be a menace to audio systems and, as you point out, isolation transformers can be a problematic solution due to the losses involved.
A better solution is to use a capacitive isolator. All you need are a pair of 2.2nF capacitors - one in the signal line and one in the ground line. When terminated with a 75-ohm load, the isolator provides a low-loss signal path in the RF region while appearing very high-Z at the line frequency and the low-order harmonics likely to cause buzz. I've attached a schematic and a photo.
The F-connectors and capacitors can be housed in a small plastic project box, such as a Hammond 1591L, Digi-Key #HM100-ND. The capacitors should be film types with a rating of 250 VDC / 140 VAC, in countries where the line voltage is nominally 120 VAC. I used Epcos metallized polypropylene, Digi-Key #495-1279-ND. A 630 VDC / 400 VAC version is also available, Digi-Key #495-1318-ND.
Insert the isolator between the cable TV outlet in your A/V room and your CATV or DVR box. That way, your A/V system is also isolated from your cable modem. For those who can't build one themselves, Jensen Transformers offers an excellent capacitive isolator, VRD-1FF (www.jensen-transformers.com). It's actually the only product Jensen makes that isn't a transformer.
I suggest checking the AC voltage difference between your cable TV ground and power line ground before adding the isolator. It only takes a small difference in potential to cause hum in your audio system, but if the difference is more than a few volts, I suggest finding the root of the problem. A CATV isolator should not be used as a band-aid to cover up a potentially dangerous situation caused by improper or failed grounding of your CATV line or your house electrical system (I once saw a difference of 50 VAC due to a failed cable TV ground!). If in doubt, call a licensed electrician and/or your cable TV company. But, even with all grounding up to code, I always use a capacitive isolator in my CATV line.
Simple, effective, yet shouldn't present a compromise to your TV picture.