Jax2, can you borrow a sub? Perferably one with a phase reversal switch. It can be a small sub that doesn't go very deep. Try it along the wall behind the speakers, with the phase reversed. I'm hoping that its response will "zig" where your main speakers "zag". Also try it along the wall behind the listening position, starting out with normal polarity. Having powered subs in your main speakers is a big advantage here, because in either of these cases you will probably need to adjust their level.
You said that your speakers were "solid", with "no boxes" - could you elaborate?
Thanks,
Duke