Amp vs Preamp which is more important?

If you have a CD player or DAC with a volume control you can do a little test to determine the performance of your preamp. listen to the digital component through the preamp and then disconnect it and hook it up directly to your amp, being careful to start with the volume turned down low. Go back and forth a couple times. Hopefully there won't be much, if any difference in sound.

If it sounds the same without the preamp in the chain then it is doing its job. It should be neutral and transparent.

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?