I, too, have a Valvetronix 15. It's got quite a few fun settings.
and a 2-tube, class AB version too, why not
Because the 12AX7 is a twin triode, you only need one. That's why the Valvetronics can simulate Class A and Class AB amps with just one tube. Unfortunately, they put a bunch of op-amps between the input and the 12AX7 so that you can't overdrive it like a valve amp.
But.... the 12AX7 part of the circuit has LOTS of potential. I've just been looking at the schematic and realised for the first time that they only run that 12AX7 at 15V (grid to plate). (for some reason, they run the plate at ground and the grids at -15V. That is despite having +15V on tap.) For stompbox use, it would make sense to use +30V.
The input stage is easy enough - a long tailed pair feeding the two grids (with out-of-phase signals). The "loads" are twin op-amps in inverting form, each with 330k input resistance. Too simple. The bias is via a fixed 5.6k resistor shared by the two triodes.
To get the "tone" of different amps, there is the A/AB switch and a couple of filters (2 x SPDT switches) in a feedback network that goes from the output of the plates to the base of one of the long-tailed pairs (the out-of-phase one).
It's all potentially "doable" in a stompbox. A key ingredient will be a suitable power supply. Maybe a 12V DC wall wart. 150 mA for the heaters, and allow 50mA for the rest.