Are running the U.S. firmware or the EU firmware? The EU firmware is volume-limited, so lots of EU users flash their Xtra with the U.S. firmware. Note that the latest firmware is non-reversible, so unless you need the Plays-For-Sure capabilities it offers, I'd stay away from it. (Some users have bricked their Zens during the upgrade, too.) A good resource for all the Creative players is nomadness.net. You'll find lots of threads on the US/EU firmware issue there.
One other thing you can try is to increase the gain on your mp3s using the freeware mp3gain (
http://mp3gain.sourceforge.net), which is completely reversible (does not reencode the files). But if you boost gain too much, you'll get clipping on your tracks.
BTW, my understanding is that the Xtra will read the replay gain info written to the file header by mp3gain, but that Media Center will not. Conversely, the Xtra is not affected by MC's "audio analysis".