Well, in hopes that this doesn't sound rhetorical or anything:
The empeg is gonna work like any other after-market head unit (more or less). What you can do is limited only by the size of your car and the amount of money you want to spend (and possibly the amount of interior work if you want to fit speakers everywhere).
The empeg is only going to give you 4 channels (front and rear pairs of left and right), which I believe is pretty much typical anyway. The average set up is probably with a 4 channel amp for the speakers and a 2 channel amp bridged for a sub. If you have eight speakers, and your amp is 2-ohm safe, you could probably hook them up to the same 4 channel and 2 channel amps (assuming there is nothing special about your speakers).
If you are an audiophile, you'll probably have speakers for your lows, mids and highs, and I'm not certain if that even include subs (but I suspect it doesn't) of which you might have 4 or more. You'll also have cross-overs to separate your lows, highs and mids into their appropriate channels. With an abundance of speaker gear, you can throw in several amps, and probably an additional power source (extra battery, perhaps even an extra alternator). Finally, for the tweaking, you can toss in an equalizer. There are other car audio toys you can install that I've never heard of before, but I think most of them may actually make the audio sound worse instead of better.
All of that stuff will work with the empeg.
What you probably WON'T have is the empeg and 4 speakers with no amps. As far as I know, the empeg is not amplified in any way so an amplifier is required.
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In your case,
The amplifier is probably fine including the eight speakers. You probably won't even have to touch them. If you want the 6 disk CD-changer you are probably going to have to keep the casette deck. At the moment, I don't believe the empeg will interface with a CD-changer (unless the CD-changer is one of those "tuner" ones).
If you keep the 6 disk CD-changer, you can run it's line outs into the empeg line ins. I'm not sure what the best way to do this is, since there are probably 4 line outs on the casette deck, but only 2 line ins on the empeg. Perhaps a Y-splitter/merger? Or just run in the front channels.
Or, the empeg can run into the casette deck if it has line ins, but then you can't use the cool empeg features when you are using the casette deck (visualizations, eq, etc).
Chances are, you won't be able to fit both head units into the same space. Either you "special mount" one of the units (some surgery may be required), or the casette deck will have to go. If that is the case, your CD-changer may have to go too (unless one of these other fine BBS visitors know of a better solution
... Tony knows everything
).
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I think the simplest solution for you might be to chuck the casette deck and CD changer (you'll never use em anyway), and just install the empeg as a new head unit. The amp and speakers should be fine untouched. If you want to install the empeg yourself, get yourself the Crutchfield guide and wiring harness if necessary (which reportedly will save you a crap load of time and headaches).
Anyway, that's my understanding of it all. It was a mouthful. Hope it is helpful in some way.
Kureg
This spelling checker REALLY sucks the big one