Check out http://www.head-fi.com in general for headphone related matters.
There is a ZOTL tube rolling thread started by Hirsh http://www4.head-fi.org/forums/showthread.php?s=&threadid=8806&highlight=micro+zotl which takes you through an elaborate journey - although it's quite long, it's well worth the read! I have not yet had a chance to try anything but the stock tubes in my ZOTL yet, but apparently this amp is quite responsive to tube changes. Good luck!
