Alrighty! I found a
Sound Meter app for android that I have no way of telling how accurate it is, but it is sensitive enough for sure. My receiver goes up in volume in increments of .5 db. When I would raise the volume 2 bumps (1db) the meter would register that 1db. So while I completely agree a proper sound meter would be more accurate, I believe this app to be up to the task for the purpose of gain matching my subs.
This was my method, using a 60hz test tone. Take notice that the feet have been removed and we have thick, soft, new carpet. Sliding these behemoths around is actually pretty easy. I took the measurements in the same room. That room being the living room. I pulled them into the middle of the living room and set my tablet up on a tripod, mic forward.
I marked the spot on the floor where the front edge of the first sub was with some tape and measured the distance between the sub and the mic. I set the gain on the back of my sub and used my receiver to dial it in right at 80db (as you can see in the pic, the mic is pretty close to the sub).
I put that one back and pulled the other one in the middle of the room and pushed it into the exact same spot, lining it up with the tape. I measured the distance from the mic to the sub to make sure it was the same, then turned the test tone on and used the gain in the back of the sub this time to dial it in right at 80db, just like the other. I believe I am now gain matched. Phew.
I sure hope someone doesn't tell me I had the height of the mic wrong or something. I don't think my ocd could take it. 
