The problem with replacing speakers in the Mach system is finding set of speakers to run at that ohm. I want to say the mach system runs at 3 ohm, which is a very odd and hard to match number.
If you are really serious about improving the sound over and above the mach system, I would recommend running a completely new system and ditching the mach all together. The mach is a good system by itself, once you begin to add components it will not match up as well as a well planned system.