Tube replacement for the Calypso?

I too went through three factory replaced pairs of Sovtek 12ax7's before I found a quiet pair. The 6dj8's in my unit sounded less stringent than the 6h23 Type III's (which I have several pairs of.) As you said, it is system dependent and in saying that, each person will have to find what suits them the best.
Each tube I tried bettered the original in one area but fell short in another. The GE's were too bright. I agree with the Telefunkin but find one that is good is the trick.
I have balanced my system out using the originals and have no complaints. You got to know that "Better" is in the ears of the beholder also. Just got to try and see.

This is an interesting thread: I also went through 3 pairs of the Sovtek's but I ended up sending the unit back to my dealer. Clearly there is a problem here: why does Aesthetix, who has a great reputation, repeatedly ship this product with inferior or faulty tubes? My first two sets were unlistenable. The 3rd set was good for about a week and then the noise came back. Perhaps I should have experimented with some other tubes, but I lost interest in the unit and returned it. Is there a quality assurance issue with this company or are they just sitting on a large inventory of crappy tubes?

I think they, like Audible Illusions, went through a bad batch of the Sovteks. It happens. They test well at first and then die.
Mine made crackle noises and some hiss. It was always one tube that went bad out of the set and always the 12ax7.
A manufacturer has to use tubes that would be readily available. Hence the problem.
Since I finally got a new set of good tubes, the preamp has been absolutely quiet and produces a lovely sound comparable to the best I have ever heard.
Aesthetix always promptly replaced them without me even sending the old one back. I now have a few good spares.

That is good to know. Perhaps I should have hung in there longer.. was so frustrated I turned around and bought a nice SS pre, the Ayre K-1x. I am extremely pleased with it, but it would have been nice to have introduced tubes into my system for more roundness and warmth.

The Ayre K1x was my next choice in a preamp if I hadn't gotten the tube thing straightened out. I don't think you went wrong with it Msaines. It is truly an excellent preamp.