I have used the AV baluns from Skywalker- they use Cat5e and I really haven't noticed frequency loss, although I use them for distributed audio, not the main signal. Yeah, they're cheap, but they have worked well. I think I used MuxLab, too.

At one house, I have a rack in the basement for the DA amps, camera DVR, garage receiver, DA preamp and an AppleTV & AVR used for the Den system. In the Den, which is directly over the rack, I have the TV, ReQuest, LaserDisc and BD player. The 3rd floor has a complete AV system, with network, cable feed (now used to bring the TV/FM antenna feed from that floor to the rest of the house) and an AV feed from the rack, which has video from the camera DVR and DA feed. No noise, no ground loop isolators, either. I'm not sure the ones with composite are still available, but pretty sure the component type are. Since the DVR feed doesn't really need to be HD, this works fine, although the AVRs up-convert. I have a CE Labs distribution amp for the cameras and DA feeds, so I don't have to deal with level losses. This house is over 100 years old and the wiring has been altered many times so it would be impossible to assure great integrity, but until the electricians ran three huge power cables next to and parallel to my audio/speaker/network cabling, it never had any noise.

Can you install cable to the sub's location? To eliminate the turn off/on issue, have you tried raising the level from the source and decreasing the input on the sub? I think the signal sense may be ahead of the level control- it would make sense to do it that way.