I think casey nailed it.

The other thing that came to mind is...

The Behringer Feedback Destroyer (BFD) is fairly cost-effective. Combined with the Room EQ Wizard (REW) software, you could have a powerful (if somewhat tedious) method to equalize each sub independently. Poor man's SMS-1. Check out Home Theater Shack forum for all the information.

But, honestly, I think that's overkill.

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