I'll be using the empeg as the source unit! So line levels shouldn't be a problem. The Pioneer will be connected to AUX-IN - it does have 4V outs which will be going to the empeg 1V in but it also has a level compensation control. I should be OK - shouldn't I?

Sounds like you've got the bases covered... I actually was thinking you weren't going to use the empeg as the head unit, so you could listen to the Pioneer while waiting for the empeg... but if that's the arrangement it's pretty straightforward. Of course since I don't have one yet, I could easily be wrong...