Here's an odd question, and it's not something I plan to do, but I'm just curious.

Lets say you have a TV that is 75 ohms coax, and you use one of those adapters to go to 300 ohm twinleads, then say plug into a cable boxes RF out via another one of those adapters.

Would that cause quality loss? Or would it cause no quality loss, if improve grounding issues, because it's isolated from ground?

It would likely help with grounding, but reduce signal strength(which if the signal is not strong enough to start with could cause increased noise).

You can definitely have signal loss that way, and some transformers are more lossy than others. Also, twin-lead cable is more likely to receive interference from other devices.

If you have grounding problems, you should fix them, not try to do by use of back to back baluns. Baluns will have some loss, but the output of a cable box or DVD player should be large enough to overcome it. Second the notion that 300 ohm twinlead will be more susceptible to outside interference.

The bad ground is from the cable company, right from the drop. They have refused to repair it because only analog cable is affected, and digital cable reports no problem.

Actually though, only my Hitachi in the sig is affected, the new TV I got (seen in the other thread on this board), is NOT showing a problem, so I don't know.. Weird.. The Hitachi works fine when directly connected to say, the Roku box, but if the VCR on the AV/RF modulator switch is connected to the cable box, even the Roku gets some hum in the picture and sound. This tells me cable grounding problem.

The bad ground is from the cable company, right from the drop. They have refused to repair it because only analog cable is affected
If I had a problem like this, I would tell the cable company to use their cable to do a personal colonoscopy.

That reminds me of when I first got cable TV, in 1984-85, and the broadcast channels had interference lines from the real over-the-air signals. They gave me a different box which did not help, then said it was probably something wrong with my TV's tuner. Oh, sure, my best-on-the-market Sony Profeel tuner is bad. I told them "Bring YOUR set over here and show me a clear picture, then I will believe you". Gee, then they got the clue and fixed it. Useless weasels! "Only analog", now there is a new weasel excuse for the 21st century. I just HOPE I never have to live in a place that has cable TV, because every day I hate that whole industry more and more.

You may also want to quote the National Electrical Code (NFPA-70), Article 820, verse 93-A to them. They are responsible for maintaining grounding the cable at your electrical panel's ground rod or via copper water pipe to said ground rod.

Your local town's/municipal electrical inspector would certainly be interested if a utility is not PROPERLY GROUNDING a service drop in his jurisdiction.