Just a thought hear but, how about this (I know nothing of electornic circuity, before anyone call's me an idiot)
How about having it like this Pre-amp tube > circuit > Tube > effect? > output
Have say a selecton of 3/4 pre-amp tubes which feed into the circuit, the circuit basically just routes the current to the Tube which is selected by the current setting on the circuit board controller, So you can mix any of the pre-amp tubes with any of the Tubes.
You could also have it so it can use 2 pre-amps together and route the signal to 2 diffrent tube types, that way you could have a larger selection of sounds.
Maybe have some kind of power scrubber/ booster in the circuitry which uses a software program, or maybe some kind of component level device to control the volts/ watts for each combination and component.
I'm not sure that'd even work, but just my 2p on the matter.
EDIT: wont the circuit flatten the tone becuase it'd be converted from analogue to digital and back again?
Is there anyway to boost this digital signal when it leaves the circuit, say with a set EQ circuit to boost it a little to get it back to a more analogue state, i.e. more dnyamic?
DOUBLE EDIT: Just thought i'd post this quick diagram to explain what i mean, it may be totally wrong, if it is, well atleast i had a bash at it