yeah I was wondering that too. For me there are very few cases where simply fronting a transparent power section with a Darkglass VMT Ultra would not be perfect 
If you wanted an actual tube there you could go with something like an EBS ValveDrive or Two Notes le Bass.
Personally if I wanted an amp at all, I would not want to have a preamp built in to the amp at all at this point and just go with a power section and interchangeable preamp via pedal. But that’s just where I am at, YMMV.