I'm surprised these guys don't get talked about a bit more around here.
Power Amp
You could argue that if all the tube mojo in done in the Axe then all you need is a linear power amp but seeing as some people prefer whatever the 'mojo' Matrix claims to offer, perhaps it would be better to look here.
I don't have one of these but from the information and the videos, it seems to me that these guys really understand how tube amps work, why they sound like they do and therefore what needs to be done to recreate that using SS technology.
Seems to me this would be great if whether you have an Axe or maybe a traditional guitar pre-amp such as a JMP1, Triaxis, etc.
In all of the tech-talk in the thread the one thing that doesn't seem to have been mentioned is that tube/valve amps are designed to work like a constant current source whereas most linear solid-state amps operate as a constant voltage source. Aside from anything else, this affects the response of the amp with the impedance of the speaker and one of the reasons why a tube guitar amp sounds 'alive' and a linear SS amp sounds dead when connect to a guitar speaker. As the guitar speaker impedance rises around certain resonant frequencies (such as 80Hz and 8kHz) the tube amp develops more power (voltage increases) into speaker in order to maintain the current. A linear SS amp does the opposite.
These Retrochannel guys seems to have figured all of this out and we can now buy a lightweight SS tube-sounding amp that nobody ever talks about