Grunge tone is about contrast: near-clean verses that explode into a thick, saturated chorus. The gear is simple and cheap — a fuzz or a cranked amp, and a big dynamic range.
Get a thick fuzz or high-gain. An Electro-Harmonix Big Muff (Smashing Pumpkins, later Nirvana) or a cranked solid-state/tube amp gives that wall-of-saturation chorus tone.
Scoop the mids (a little). Grunge choruses often scoop the mids for a big, wide, aggressive wall — but not as extreme as metal. Keep enough mids to cut live.
Lean on quiet-loud dynamics. The signature move: clean or near-clean verses, then slam the fuzz/distortion for the chorus. Use your volume knob or a footswitch — the dynamic jump IS the sound.
Try drop or alternate tunings. Soundgarden and Alice in Chains loved drop-D and alternate tunings for heavy, loose, droning riffs. Detuning adds weight and menace.
Keep it a little raw. Grunge is not polished. A slightly loose, raw, even sloppy edge is authentic — don't over-tighten or over-gate it.
💡 A cheap guitar into a Big Muff into a loud amp gets you most of the way — grunge was built on affordable, imperfect gear.