Aesthetix Calypso How many versions?

I've seen some people advertising their for sale Calypsos as being the "latest version with the ultra quiet resistors" Why, how many versions does this model have? Or are they all the same? Should I look for the latest version or does it not matter? It seems like the "newest model" ones sell faster, so should I get the newest model so I can resell it quicker if I don't like it?
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In my research, it seems there has only really been one change, which had to do with resistors. There were some noisy stock tubes reported for awhile, and that now seems fixed. There were also some hum problems reported with Ayre amps, but Ayre changed their grounding scheme and took care of that, I don't believe the Calypso changed anything with regards to the hum when used with Ayre. I would be interested to hear from anyone more knowledgeable on whether there were additional changes or versions, and maybe what serial number started the resistor change.
Does anybody know what is the benefit of ultra quiet resistors? Quieter background or less hum maybe? Probably Jim White can answer this better.
Probably the same mod I just did to my Aesthetix Io, swapping most of the Mills Military for Roederstein precision metal film resistors.

The Roederstein's are the standard now. Quieter, lower distortion and superior resolution.

Part of the upgrade on my Io was a slight correction in the RIAA curve. Obviously this would not apply to the Calypso as it is a line stage only.

Those with Io's more than one year old, be aware of this upgrade. It's extremely audible and very inexpensive.
There was one early change to a few resistors that would/could become slightly noisy. This was done several years back after the first production models hit the market. I think you're good to go with any serial number above about 2650 or so. Of course, Jim White was good about updating the units that did have issues. They too would be fine.
There has been no other changes that I am aware of except Aesthetix did change tube types to help cure a noisy tube problem that seemed to be fairly common for a while.
The Calypso is a great preamp for it's cost even new.
I certainly wouldm't be afraid of a unit several years old.
The Saturn series (Calypso, Rhea and Janus) was designed from the outset with Roederstein resistors. The Mills resistor issue had already come to light by the time they were designed, and none were ever shipped with Mills resistors. Only the Io and Calliso needed to be updated with Roedersteins.

There have been minor changes in production of the Saturn series over the last 5 years, most of those occurring during the first 2 years of production. None are of great significance. If you call us with a serial number, we can advise if there are any updates available for the unit.

Thanks for all the strong support! It is greatly appreciated.

Jim White