How far are you from the rear wall (the wall behind the listening position)? A distance of around 4 feet will attenuate (reduce) the bass peak you are perceiving, if that would be practical. A distance of around 7 or 8 feet will add to it (while also attenuating frequencies around 40 Hz).
Regarding subs, if I'm interpreting your post correctly you are envisioning using a sub which provides high-pass filtered outputs, which would be connected to your power amp. I suspect that using simple XLR-to-RCA adapters at the power amp inputs would work ok, although probably with a 6 db gain reduction. If not, a suitably chosen Jensen transformer, ca. $250, would certainly work ok. However, I am not aware of any subs to suggest which sell for less than $800 used, that would provide **high quality** high passed outputs with a high enough crossover frequency, and that would be suitable sonic matches for the Maggies. Also, I suspect that in order for that approach to be helpful with respect to the bass peak the crossover frequency may have to be set too high to be optimal.
Good luck. Regards,
-- Al
