if you google prs mt15 tube layout, you find a link to a thread on this forum and there is a layout PDF uploaded.
Bit of a weird layout, because furthest from the power tubes appears to be a phase inverter (usually closest to the power tubes as it's part of the power section). Those usually get worked harder than any of your preamp tubes and can play some games when they go bad, so maybe worth swapping that out with a new one to see if it helps. Depending on how loud your crackling is, and whether it is present all the time (ie, when you switch between channels) you can narrow it down to v1. Because that's the first two gain stages, and noise there will be amplified by subsequent stages. That also means if it only crackles on a drive channel, it might be a later tube and not your v1. But the easiest way to figure it out, if they're all the same tube type, is to swap each one out one at a time with a known good/new spare.
Rustling noises IME are usually a preamp tube, although I have had popping and farting and carrying on from bad power tubes too, fwiw