Thanks for help everyone.
little more background. These houses are built with 6 prewired speaker/j boxes in the ceiling as an option. Or at least my one neighbor has the same jbox/speaker setup but without any actual speakers. The houses built in 2012.
The in wall sub was the only theory for the Sub out wire I came up with, it’s even labeled “Sub”. All the other wires coming out of the walls work as they are labeled. I have not thought that perhaps that wire just runs to another j box. I’m going to open all the boxes in the room I can see to tell if it comes out elsewhere, but if it does its in the j box and doesn’t have its own box and plate. There is no grill for the in wall sub. It’s buried behind drywall, I can feel something in one of the bays. Part of the wall is exterior so it’s insulated, and I do renovations so I have a decent feel for when mechs are behind drywall, but it’s an art and not consistent. I never hooked a multimeter to it as I know I’m sure it’s not hot, I tested for a current with a hot/cold tool.
The attics here are blown in insulation, and it does not look like anyone has made it through the hvac straps and insulation to install these so I’m sure it was done from the room itself. I don’t see any obvious texture/paint patches so I think the wires went in during the build.
Asked the old home owner and no reply.
I’m thinking of just getting one of those $25 basic 100w amps from eBay and seeing what happens as a low setting.
Have a new baby and I need to disconnect all my A/V speaker wire to get a photo and open up the plug so it may take me a few days. I’m going to put extensions on the wires so I don’t have to do that every time I pull my box out.
Thanks again
