I had one sub broken for awhile (I normally run two) and when I had one I had to really mess around with placement, crossover point etc. got the other sub back so back to two and the System just snapped right back into place. Sounded even and right again. All I can figure is only using one was making lumps in the frequency response and sometimes it sounded fine and other not so much.
now 28hz is pretty low and might not benefit as much. I was running 60hz at the time notes above.