abstract7, no heresy charges here! :>) yure absolutely right - bass traps or system-equalization will be the simplest, most effective cure.
re: moving the speakers around - try off-setting the speakers a bit - i.e.: have the left speaker either closer, or farther from its nearest side-wall, than the right speaker... or, even cock the entire plane of the speakers - i.e.: have the speakers different distances from the rear-wall, even tho the speakers-to-listener triangle set-up remains the same.