Hey Duncan, the 580s and the X-Can are a great match . Who cares if the X-Can isn't the greatest design, it works great with with the 580s. It's all about synergy.
You could buy a more expensive amp and headphone, but if they don't work well together, what's the point? I've bought components that I thought were great(and expensive) that sounded good at the store, but that sounded bad with my particular system.
I saw this ad in a Acoustic Sounds catalouge(they make some of the best recordings and sell other CD/LPs, etc.):
"The most tireless listening combo available in headphones. This dynamic pair(X-Canv2 & 600/580) is equivalent to or better than most costly, top of line speakers."
-David Baker, engineer, APO records
Okay, maybe that was too hyperbolic and just advertising propaganda, but we're not taking about Bose, we're talking about a tiny record label and audiophile store, for what it's worth.
If you keep the Can', change the tubes at some point though. And if you have to get rid of something, I'd get rid of the Sony phones first.