Mitch, I have owned the Calypso for about a year now, and think it is stunning. It replaced my ARC REF II mkI, and I've never looked back. In my experiences with tube swapping I settled on Ediswan 6922's and Telefunken 12ax7's. I actually liked what the Ediswans did for the sound better than what the tele's did. The eddies are very smooth with an incredible mid-range. I also plan on experimenting with the Mullard/Brimar BVA 12ax7's very soon. Please let me know if you have any other questions. I hope this helps.
Darrin
Darrin