Hey, no problem on the info, but it's still just based on how I assumed the Stiletto's preamp was designed.
I have worked on newer Mesa's and they are not very easy to modify.
Knowing how they cram circuits i think it would be nearly impossible to add a Fender tone stack to this amp. The traces are too delicate and most times too difficult to trace. You can mod a bunch of components, but it would be a ton of work and serious knowledge to design a new tone stack what incorporates the relays and switching transistors. If there is one thing Mesa can make complicated is it's switching systems. You would need a schematic and a truth table to see what gets switched in or out and when. If you want a Fender type clean, the RK II uses a Fender tone stack and even goes as far as using carbon comp resistors in the clean circuit. As an added benefit, you can use two EL-34's on any channel for a hot rodded Marshall vibe. If you want to go with 2 6L6's and 2 EL34's you will encounter a wicked ride of bottom end and the cut from the EL-34's.
As far as the Siemens EL-34, i have Mesa, GT and no label versions of each. I have 12 in total and I have used them in Marshall's and IIC+'s and the RK. They are a superior tube, but you may want to keep them under 450 volts or they tend burn out quickly. SED's seem to give a very close tone and can handle the higher plate voltage with a much longer lofe span. Like the STR-415's I only use the Siemens for special occasions. I'm a wuss who dreads using an expensive and nearly extinct resource unless I need to. For recording a certain tone I would use them, but for the punishment of gigging, I use the current stuff because I would probably have a major bout of depression if I killed a NOS tube.