The first thing you should do is upgrade the power supply on your X-CAN (called the X-PSU). Good luck with that, since it's so many generations old. Musical Fidelity currently has the V-Can, which, contrary to its nomenclature, is not 2 less than the X-Can. They also have the M1HPA. If you're interested in buying new, I'd go with the M1HPA.
And yes, Pinkie (rock grotto) has some upgrades, including a DIY power supply, if you're handy with a soldering iron.
Also, if you haven't yet, get an aftermarket upgrade cable for the HD600. I like Cardas, but pretty much anything is better than the stock cable.
Also, (I probably should have asked this first), what do you mean by 'better'? Do you want more detail, more euphony, more accuracy, more bass, more rawk? What volume do you listen at? What kind of music? What's your favourite colour? <--Note British spelling.