This looks like a great build. Good luck!
A couple of questions...
Are you doing the designs of the circuit tweaks yourself? Are you self taught or do you have a technical background? I've got a basic understanding in a paint by numbers type of vibe, but I'd love to get a better understanding so that I can hopefully put together something "unique" in the future...
Are you building off a basic kit or sourcing components individually?
I'm a very technically proficient person in general but I don't know much about circuits (besides the basic sort of stuff). So basically, I'm looking at the JTM45 and 1987x schematics, noting the differences between them then seeing what I need to do to put both flavours into a single amp. It turns out that there are only a small number of big changes between them. Of course, the 1987x (50 watt JMP Marshall) uses EL34s and the JTM45 uses KT66 so I had to choose the one or the other power section and since I could get a 8kΩ output transformer for cheap I went with KT66 so this amp will be a JTM45 (with a slightly stiffer power supply) with both the Bass-spec and Lead-spec tonestacks. There are other amps like this but they're few and far between and super expensive. Its going to be a Marshall and if I do this right, it'll sound like a late 60s plexi, I'm not aiming for something unique, I'm aiming for the holy grail of rock guitar amps ?
As to kits and parts. I bought the parts from Valvestorm, taking most of their basic JTM45 parts., i.e. sockets, pots, capacitors, switches, wire, nuts and bolts etc. and adding the extra ones I will need. Its not really a kit, nothing is pre-made. The chassis has been punched but that's it, I'll need to measure and cut and drill the turretboard, work out my own layout, have a box made etc.
I bought the turret material and turrets and standoffs from elsewhere and I still need to have faceplates made (I'll know what they need to look like once I've decided which switches I need and where to put them).
The trick is that you should buy as many of the parts from the same place, to save on shipping, plus I got friends to haul back everything from the states and the UK for me. This will probably save me around R4000 in shipping for the entire project (And with the exchange rate this helps since its already quite expensive)