Welcome, no more. As John Bridgman suggested, that elevated area around 35-45Hz appears to be the result of the nearly square room dimensions causing two adjacent mode peaks(560 divided by 14.5 and 12.5 feet). Both of those modes would be minimized by placing the sub near the center of the room, but that's not likely to be practical.
The equalization on the 3805 has been reported to be helpful to some and not to others, but in any case it doesn't equalize the sub output, just the other speakers, so that wouldn't be helpful on the 500. Besides positioning, the BFD, which can be connected to equalize the sub output can likely smooth out the lowest bass.
