The typical "tried and true" DIY Jose-style mod in a Marshall circuit:
Cascade all the stages
0.68uF/2K7 cathodes in stage 1 and 2
0.68uF/820R in third stage, raise to 1K for more clarity
Stage 1 and 2 couplers to taste
220K-330K plate on first stage, dealer's choice. Can bypass with a small cap to tame oscillation
Two gain controls with bright cap on first
Tone stack to taste. Either stock 33K/470pF-560pF values or play with whatever 47K/39K/etc. slope
NFB whatever you like, but for a 2204 47K/8ohm tap works well to start
Depth control with 2200pF-4700pF cap, whatever sounds good
Filtering and voltages to taste
Pre-stack master volume with zeners or transistors wired as zeners to taste. Don't forget the resistor and blocking cap. Anywhere from 18V to 12V. Symmetrical or asymmetrical. MPSA06 or 2N5401.
This is what everybody and their momma's been building for 40 years and charging people $6,000 for. You're welcome.
