Assuming your power amp has no trouble driving your speakers and it doesn't do anything really bad sonically, I would say that the preamp is easily more important. Just to give a rough example, if I had $5000 to spend on both pieces, at least $3000 would go to the preamp. Keep in mind, though, that this is very general info, and in audio, there are always some exceptions.
If you're on a limited budget and can't afford a really good active preamp, a passive unit is a great alternative. There's nothing worse than having to suffer with a cheap active line stage. Especially if you're new to high end. Chances are that you won't figure out what the problem is until after you spent a lot of money.