Well, 8" subs aren't exactly huge, and tuning the cabinets to 40 Hz displays a real limitation in bottom end response. I'm guessing 32-35Hz.
Generally it's advised you set your sub levels at + 3 to + 6 dB above your main speakers.
Sub placement is also very critical. Make sure your subs aren in phase, both with each other, and your system. It's easy to get a lot of cancellation, room modes and nodes are a pain in the but to work around too.

Could be a receiver setting...verify you have your speakers set to "small" and your LFE or bass setting to "sub" only. Set the cutoff/crossover to 80 Hz or so.