When using parallel compression, do I use eq on the original signal or leave it totally raw? I know 1 channel is compressed and 1 is left original, with no compression but does that mean i shouldn't eq it?
Parallel compression works like this: you take your stereo track, and send it to a compressor that has the ability to mix in some of the uncompressed signal with the compressed one.
You turn the mix to 100$ compressed, slam the crap out of it with compression (if that's what you want), and then change the mix to something closer to 50%. The effect is a highly compressed and dense sound that still has the attack characteristics.
So, to answer your question, no you wouldn't usually EQ one or the other.... BUT you certainly can try if you actually take a duplicate track to use as the compressed part. The only issue becomes Phase alignment, for example are things like the phase of the bass of the kick in alignment so that one part of your balanced compressed/uncompressed doesn't cancel out the bass of the other, leaving you with a rather flaccid sound in the end.
Fun stuff to play with. I'd also suggest taking a look at M/S processing. You can do some really cool stuff once you cross that line.