Before you do anything,
I would check the crossover controls on your amp, ESPECIALLY the "high-pass" control of your front speakers, and make sure you don't have it set too high, or have the X1,X10 switch pushed by mistake. Especially with those big mid-bass speakers, your doors should crossover pretty low, like 100, if that's as low as it will go.
I was just looking at a 406 manual here; I imagine it's similar:
http://www.mcintoshlabs.com/downloads/product_manuals/mcc406mom01.pdf