Hey RG, maybe think about it like this, tubes add gain, so do opp amps (in some ways the transistor (SS) version of a tube??). They all come in different shapes/sizes and amounts of gain they bring to a circuit. Eg you can use an opp amp to power your Fx/Aux send(s) on a desk to ensure each send gets good level when you split the input signal to them. So from what MJMP is saying, this circuit is largely driven by SS opp amps with tubes thrown in to warm it all up (kind of hybrid circuit, not uncommon BTW). So if the tubes aren't doing that much of the distortion/tone, changing them will have less affect. (it will have some though). Now you could change the opp amps to reduce squish, but that's not as easy as say going to lower gain tubes. So another approach is go with high gain tubes (again the Mullards would be fine or whatever else you have around) and adjust it such that you have maximum tubes in the circuit. Eg, my Ivory 5001 hybrid valve 4 ch preamp has SS circuits with a tube to warm them up. winding up the ch input puts more signal to the tubes and winding down the output balances it out (more tube less SS). Seems your gadget is similar...