Amp vs Preamp which is more important?

Which is more important, you right foot, or your left foot?  The preamp certainly has the more complex job.  I sidestepped this question by buying an integrated amp and probably gave up the ability to have the best of either.

Amps can sound pretty good without a preamp. Preamps don’t sound at all without a power amp. Headphones excepted. :)

IF the amp and speakers are decently matched AND if the amp is decent -- the preamp is more important.

@ghdprentice put his finger on why: "It takes the really small signal and brings it up to the level the brute can handle it. The preamp is the heart of the system… it brings consistent sound to a system."

Both are important and making sure they sound good together is equally important. Having said that, I would suggest you upgrade your preamp.  If you like the sound of you ARC 16l and your PX-25, you might move up the ARC line.

How much were you thinking of spending?

Preamp. Garbage into your amp only produces much louder garbage out!