I don't think aftermarket speakers will work very well because they are mostly 4 ohm and the Mach system speakers are 8 ohm. My suggestion is to take the opportunity to replace all of the mach garbage with nice aftermarket stuff.
Agree 100% that's why I still have the Mach 460 stuff. I'm buying cams this spring and nitrous, but there will be nothing left to do on my car for performance unless I spend big money on a supercharger which is unlikely or do some significant suspension changes. After the cams and nitrous, I plan on getting a nice head unit with an LCD display, a decent 4 channel amp, and component speakers. I currently have a MTX sub in a custom mustang box along with a MTX mono block amp.
Someday I'd love to build an aluminum high compression engine with ported heads a nice cam, etc... along with MM k member, MM T/A, MM PHB, and coilovers... Will probably wait until my son is old enough to do it with me. It will be a fun project for us, but it will be a while.. he's only 18 months old
I replaced my front midbass speakers with Elemental Designs 6.5" subs. Had to use a 6x8 to 6.5" adapter, but they fit just fine. The subs are dual 4 ohm, so wired to 8 ohm each and they work perfectly with the stock amps.