I (and several "high end") installers have been trying to figure out the cause of a very strong system resonance between 650 and 1000hz. I have tried head units from Alpine, Nakamichi, and Sony; ADS, Dynaudio, Focal, and Kicker speakers; Nakamichi, McIntosh, and JL Audio amps; PPI and Phoenix Gold Xovers; and Audio Control and Eclipse EQs. The doors had been modifies to fit 6 1/2 3-way systems. Cranking the EQ way down in the offending bands helps some, but that, of course, messes up lots of other things. I am at a loss. Has anyone else experienced this problem? ANY help much appreciated.