Somehow your request has slipped through the cracks in the Internet.
The Muff tone stack is lossy, needs some kind of boost after it, so...
1. into the input — An unusual choice, but lots of gain compensation afterwards.
You could wire up the Muff tone on the back of a pot, freeform ala
BEAVIS:
or with a bit of vero:
You could have adjustable mids (click "BEAVIS" link above) if you include another pot (use 9mm pots and Davies1900-style or similar knobs if you really want to squeeze it in a 1590N1).
Not sure how feeding the Powder Blue circuit a prominently scooped signal will affect the rest of the downstream sound, but it's surely worth experimenting with.
2 . before the treble control — Bump up Powder Blue's R7, perhaps
3. after the treble control — R7 again, increase it or even replace it with a trimmer to bump up the gain as needed,.
4. before the output — You'll want a recovery stage there as well as the Muff tone stack.
5. somewhere else — Sure, another build and stick on a SWTC before the master volume.
Maybe the easiest way would be to buy PedalPCB's PedalBlock Tone Control,
and then you could try it in all the positions you've suggested with alligator clips/sockets before committing to the preferred method. Like you could just take the PCB's main out and send it to the Muff Tone pot, may or may not still need to bump R7, depending on the gain drop from adding the Muff Tone Stack.
Check to make sure you've enough room between the Bass and Treble pots, but that's where I'd stick the extra pot. That might force the toggle north of it to be offboard wired and moved further up between the knobs. If you use a 9mm pot, it should all fit, but getting the PedalPCB PedalBlock onto a 9mm pot might be... interesting.
Thus far, I've only tackled the problem of adding the Muff Tone Stack, but you want a boost, too. You're running out of room in a 1590N1, but it's still doable, depending on which boost circuit you choose.
Oh, and to get the midhump in the Muff Tone Stack you might want to go here and play around with the numbers: