Help Troubleshooting Sub with No Sound

Hey everyone, I'm new here and a novice with car audio. I recently installed a powered subwoofer (integrated amp) into my Subaru Outback using the factory headunit and speakers, and it was working well, but suddenly won't work anymore. I had not changed any connections, settings, or anything else between when the sub worked and when it stopped working. I am looking for advice on troubleshooting steps to take. Please have a look through the details and let me know if there are any things I can try. Thanks!

The Problem:

Subwoofer will not produce any sound even with max Gain (LPF set to about half), audio source (bluetooth smartphone) equalizer set with bass to max and volume all the way up). Subwoofer used to make an audible "pop" noise when turning the sub on and off when it worked properly, but no-longer does so. Note that I used to have a similar issue with the same sub in my other car (aftermarket headunit, Alpine 4-ch amp, aftermarket speakers), so my money is on something wrong with the sub itself.

EDIT: Something weird I just noticed. The gain dial on the sub ranges from 6V-0.1V. That is, fully counterclockwise is at 6V. Is my Gain dial reversed from the normal "Clockwise is more gain"? See attached pic.

System:

-2001 Subaru Outback Base (Factory Headunit and speakers)

-PAC SNI-35 Line Output Converter (to use RCA cables with factory headunit, I tapped into the front speakers)

-MTX RT8PT Powered Sub (120 W RMS)

Connections:

-8-Gauge power wire from car battery (with inline 40A fuse)

-Ground from sub to frame, ring terminal soldered onto 8-Gauge ground wire, screwed with sheet metal screw into unpainted frame (NOTE: I did not sand the area because it was unpainted)

-Remote turn-on wire from headunit 12V ACC to sub

-Twisted Pair RCA cables from LOC (behind headunit) back to sub (NOTE: one female RCA jack on the sub was broken (internal plastic was missing and brand new RCA cables would not plug in to only this jack), so I soldered in a new RCA female jack, and all was working well).

Proposed Troubleshooting Steps (Green is what I've already done):

1. Verify inline fuse (in engine bay) and fuse on sub are good (both visually inspected to be ok. Subwoofer Power LED turns on when car is on, and off when car is off).
   
2. Make sure sub speaker is not blown with 9V battery test (connected 9V battery to 12V+ and Ground terminals on sub and the speaker pops out just a bit) (I had also checked Resistance across the sub speaker itself and saw a non-zero, constant reading, so I assumed it was not blown). May need to recheck these.
   
3. Verify amp on sub is not in Protection mode (Protec. LED is OFF)
   
4. Verify Gain is not off on sub (Gain set to max)
   
5. Using DMM, test that subwoofer is receiving 12V+
   
6. Using DMM, play something like a 50 Hz test-tone and look for AC signal coming through both RCA cables (remember the RCA cables are brand new, so I don't think they are bad)
   
7. Sand ground area on frame and reattach sub ground terminal, make sure connection is snug
   
8. Check front speaker to LOC connections behind Headunit and make sure they are still intact (they were soldered and heatshrinked) and not shorted
   
9. LOC has adjustable gain for each of the two channels, make sure they are at least 50% (though everything was working before with whatever the current LOC gain settings are)
   
10. LOC has two brown ground wires coming out of it. The instructions say "Connect the brown wires to ground only if there is no sound from the amplifier". Currently the two brown wires are not connected to anything (and this configuration worked fine before, so I'm not sure why this would change anything).