Could be a wattage problem. It takes more power to push bass then treble. Have you checked to make sure they are getting sufficient power, or that they have clean connections?
If you run a speaker with lower impedance then the amp can handle you'll have problems with the sound. If you run a speaker with a higher impedance then the amplifier you'll just get less output. The solution would be ditch the mach 460 amps and run an aftermarket setup, or find some 8 ohm speakers.