I have a head unit that only has two RCA sets - front speakers and "rear," which is interchangeable between subs or rear speakers. Likewise, the rear speaker lines, not RCAs but actual things to which speakers hook up, can go from true speaker lines to sub output only for amps with high line inputs. Keep this in mind for a later paragraph.

I have one Infinity 4ch amp that takes in both front and rear RCAs and powers all my cabin speakers. Then, it has an RCA out which feeds some Diesel Audio amp that I use for subs. Not too long ago, my rear speakers cut out so that only the fronts work, and I also have no bass anymore from the subs. I can hear that the rear speakers are on, but they're just not playing anything. Haven't checked if the Diesel Audio amp is on or not.

Speaker setup so far as set up by WakefieldL and myself:
Front: 6.5" components by PowerBass powered by two of the Infinity'* channels
Rear: 6.5" components and 6x9 Infinity Reference speakers powered by the other two channels on the Infinity amp
Woofers: 12" Sony Xplod subs powered by the Diesel Audio amp

My original plan was to use three amps so that I'd have a separate one for the Infinities. In fact, the Diesel 2ch amp was going to be used for the Infinity speakers while I had on hand a Rockford Fosgate 2400W amp for the subs. Realizing that the Rockford is way too awesome for my tastes in bass, I set up the speakers as they are now. Do you think that running 6.5 components and 6x9s on the rear channels made the Infinity take a poop? And if so, what of the Diesel?

Rear speaker lines paragraph:
I have a Pioneer mono amp in the Alero, and I'd like to feed it my rear speaker lines so that it can run a sub or two. That would free up the Diesel to run my 6x9s (or I could take them out), and I'd like to fix up the 4ch amp or replace it. Does that sound like something that should be done in favor of other options, once the 4ch amp is fixed, or is there something else you'd recommend?

Thanks!

does your sub amp have high and low level inputs on it? In other words can you use speaker wire as your input signal?

Ok, update! I fixed everything, and all the amps are back to normal. I'd still like to fix up things so that I have individual control over my sub and my speakers. All amps have both high and low level inputs, but I've only been using low level, RCAs, so far. I'm going to use high level input with my head unit knowingly putting out only a sub signal over the rear speaker wires, and then I'll have my RCAs for individual front and rear sets of channels.