Why???
It would be virtually impossible to reverse engineer the software. And the AVR isn't powerful enough for DSP.
Do you know what the processor is for? It could be used for effects and in that case it would make more sense to build a separate effects box/pedal. Or it could be used to simulate the sound of different amplifiers. That's more difficult because you'd have to know the characteristics of those amps, plus the characteristics of this particular amp.
[quote]I told him that valve circuits are past technology and he replied yes, but they offer much more good sound quality.[/quote]Most guitar players do prefer tubes and they usually have a preference for a particular amp/cabinet. (just like they have a preference for a particular guitar & pickup).
I'm pretty sure you can make a solid state amp that distorts "exactly" like a tube amp and there are "modeling amps" that simulate a variety of existing amps but "good luck" convincing a guitar player that it sounds the same...
Overdriving is clipping, but tubes & guitar amps tend to "soft clip" (something like a limiter or analog tape saturation).