I'm thinking about wiring my empeg unit up an existing head unit. Am i correct in assuming that i'll have to use the same 1v Line Out i use hook it up to my stereo at home ?
Or can i actually use two of the 4v car outs (lets say the two fronts) to wire it up to the head unit? Wont i loose some of the sound ?
Or are there Head Unit with quad in's?

You use two (fronts or rears) of the four 4V outs as the
'home' RCAs aren't docked in the sled. I doubt that there's
any head unit that would have a problem with 4V outs fed to it.
In the worst case, just don't turn the empeg up all the way.

There probably are head units with 4 inputs but consider this:
The source material (songs) will only have 2 channels.

The 4 outs just give you the opportunity to apply different
amplification and equalizing to front and rears...

hmm... so in fact all i'd loose the fader.. and the ability to set the equalizer for front/rear seperately...
that's it ?

The empegs fader, yes, though the headunit, which drives the
speakers (or amps?) probably has its own fader control.

Equalizer - yes you'll lose the ability to separately affect
fronts and rears. But you can lock it in 2 channel mode and
get 10 bands per channel instead of 5 - though 5 parametric bands probably are enough for most any user...

I can't think of anything else you'll lose by going through
a head unit - any one else? You do gain the ability to have
sound in the car even without the empeg.