What's the likelihood of purchasing an adapter (or splicing the cables to RCA audio plugs) and it would work without issues? Anyway I could "test" for interference using a tester?
I do not know. Guess you can try it.
Can you get to the wall from below or above or can you reposition the subwoofer location and use cat5e cabling?
Here ran microphone shielded cable from the 2nd floor to the first floor for an AVR to AVR connection many years ago. It was noisy. The wire though starts next to the AVR then down to the basement, then up via a chase to the attic and then down to the master bedroom.
I switched over to a catxx to RCA balun and the noise went away. I don't use it any more these days. I did also use an cat5e audio balun for a friends dental office. The run was from the back of the office computer room to the receptionist section. Probably some 75-100 feet and I never heard any noise on the lines.
I did also utilize same microphone shielded cable for the subwoofer set up in the family room. I hear no noise on the subwoofer but it is a short run compared to the basement to 2nd floor run. Initially too ran speaker wire for a in wall set up and didn't want to cut in to the wall.
Googling
Can I Run Line Level to a Sub Over Speaker Wire?
First hit was from a 2005 post
Question:
Hi there,
I'll be getting a new subwoofer soon, and I'd like to place it somewhere else in my room, away from my system. There's an unused terminal for in-wall speaker wire there, so it would be great if I could run the line-level output of my receiver to the sub over this speaker wire (using phono-to-bare-wire adapters.) The run would be about 10 feet over 12-gauge wire. I figure the wire comes within 18 inches of some power lines.
Is the lack of shielding likely to cause problems in this application? Or is it just a try-it-and-see situation?
Answers:
I'd say try it and see. Obviously it is not an ideal solution. Part of the problem is that the line out signal is probably high impedance and not well suited to driving a speaker cable properly. A lower impedance signal is much more suited to drive a speaker cable without problems. The fact that you are feeding a sub may work to your advantage since there should be very little high frequency content that will make it through the sub. I'd think most problems with this type of transmission will manifest themselves higher in the spectrum. Of course hum pickup, if any, will be heard through the sub.
Another problem you may experience may be that hanging the speaker cable on the line output may cause problems with the receiver. It is certainly possible that this could pick up rf or other garbage which could get back into the preamplifier's output section and deteriorate the overall sound to some degree. Then again you may not have any problems with this.
This certainly would not be the method I would try, but it may just work. If you try this, please report back and let us know how it worked for you.
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If you were using a Rel sub that that has the hook-up that can run off your L & R speaker posts, it could be ideal if you don't pick-up noise in the wall? Couldn't you use the wires to move the signal over to the sub? Couldn't ya?....
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With subwoofers, you can quite satisfactorily use a long run of shielded coaxial cable with a converter at each end to match with the RCA connections at the sub and the preamp. I know this is not as convenient as the wiring that is already in your wall, but it is handy and inexpensive.
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Btw, if you do have problems with receiver degradation, a possible solution might be to put a small rc network across the bare wire adapter you make. This can be a series resistor on the positive leg followed by a capacitor across the output of the resistor and the negative leg. This will shunt the high frequencies and may lessen crap getting back into the receiver. Normally this would be a bad idea because the highs will suffer, but since there are no highs needed in the sub it may be an option.
