I had the same problem, mach 460 system. All speaker worked, then replaced them with brand new speakers, and now only the back two tweeter slots work. I pulled all of the fuses in both boxes, nothing. last thing for me to check are the two amps in the back. By what i've read about the mach 460 system, its a bunch of crap. So what i'm going to do, is scrap the two amps and then buy a new 4-channel amp and hook that up to all of the speakers. If is the amps, i've read that the fuses for them are next to the plug in closer to the front. When I looked, I didn't see them. So I'm not sure if they are visible after you take the amp off, or if they are inside the amp.