Pass First Watt models ... confused

I'm curious about the First Watt products. I'm a very happy owner of a Pass x250.5 AB amp but interested in experiencing full class A. No plans to sell my less than 1 year old x250.5, and I'm not rich so I won't be sinking cash into the exorbitant prices of a new XA series. A used Aleph is a possibility, but those are getting fairly old and I wonder about maintenance issues.

Therefore I am looking at the reasonably-priced First Watt line from Nelson. But I'm quite confused by all the different models. Can anyone help clarify this puzzle for me? Thanks.