if you can "barely hear" the midrange, you could have mixed up the phase somewhere along the line...

Oooo, good call.

Yeah, if bass and mids seems mysteriously absent or muddy, turn the balance control to the left and the right. You should hear a linear reduction in overall volume and timbre as you move the balance to one side.

If instead, you get MORE bass and mids as the balance is moved to one side, then you have at least one channel wired out of phase (the two sides of the car are canceling each other out on the mid and low frequencies).