Can it be that the mic picks up a strong reflected sound? That can double the delay. May be to try to point the sub at different directions, turning it aroung?

Ten feet difference is about 10 millisecond delay. If my memory is not wrong even a simple filter would give a delay about 1/(3fc). So if it's set at fc=80Hz, then the delay would be only 4ms. Does your sub has an option of bypassing crossover? Might try that too.

Regards,
Gena