I have spent quite a bit of time and money testing tubes on the Calypso. First off my experience with the stock 12AX7 Sovtek tubes was very bad. Ive had three sets and they all were very noisy. I tried a fourth Cryo-treated Sovtek and it was better but not to my standards. I also tried cryo-treated GE and WE Jan N.O.S. Tubes and found them to be a good step forward at an adorable price. The WE was a bit slow and thick for my taste but excellent midrange. The GE was better balanced but not as good of dynamics. All of the cryo-treated 12AX7 tubes came from www.tubeman.com and were well matched and very quite. I then tried some tubes from Upscale Audio, Kevin is maybe the most knowledgeable in tubes and has the highest standards of any. He also has the highest prices, but you know what your getting, THE BEST. I tried a Philips Holland 1959 12AX7 under the Pope brand name. Same tube as the Bugle Boy. This was very dynamic and extremely extended with very tight bass. It tends to the treble a bit more than all the others and would be great if your system is slow or dark. The Mullard Britain Military 12AX7 a.k.a. CV4004 is silk with punch. Everything about it is perfect. Pace, dynamics, bass, extended highs, transparent, dead quite and the most lush midrange available. It is not cheap at $80.00 a tube, but its well worth the cost. There were three pair available at Audiogon a couple days ago for big discounts. All were bought from Kevin. The most wonderful tube was a Telefunken 12AX7 but I am still trying to find a quite pair. These tubes have for the most part been exhausted and the ones left are less than perfect. Kevin does have them available but the cost is too high for me.
Now that was the 12AX7, I also switched the 6922 with NOS tubes. I got one set of Russian 6H23 Type 3 from Kevin and a pair of Cryo- treated Russian from Tubeman. I liked the Cryo sightly better, but for the price I would go with Kevins.
Tubes really mattered a lot in my system, Sony SCD-1, Plinius SA-102 amps, Dunlavy IVa speakers and Nordost Valhalla cabling. It may be that a less revealing system the weaknesses of the Sovtek are not so apparent, but in mine its huge.
As far as other dealers quarry at Audiogon, there are a ton. Sam who sells alot on Audiogon seems very reputable, he is working with me to find an exceptable Telefunken.