According to the progression of eBay auctions and Carver's comments in various places, this succession of tube amps started when he cleaned out his warehouse and found a lot of parts he was saving for no particular reason. He had a lot of fun building some new designs, one thing let to another and he was making his own transformers and so forth.
More than anything, he does this because he loves to do it. Why tubes? Maybe it has to do with the fact that he's pretty well plugged into the community and knows that many of the folks with deep pockets and collector types are into tubes. They are also pretty simple to design and build, relatively speaking.
I think he even had some SS amps that had a user switchable mode that emulates the even order harmonic structure of tubes.
Most of the Sunfire designs have dual outputs with one of them intended to offer the "tube sound".
The current source output on the Sunfire has a 1 ohm resistor connected to it, which supposedly gives you a more tubelike sound. I guess it raises it's output impedance?
I think the same applies to most of the Carver "t" series SS amps, but they only have the one output. Most of the Carver designs can run to theoretical "0" ohm loads for brief periods, (peaks), many are rated to 2 and a few are rated to 1. So adding a 1u resister doesn't change much in that regard.
In the old days, Carver was often quoted as saying that the real difference between tube and SS was distortion, with tubes having more. Whatever it is, some people really like the "tube sound".