Okay, I found it, here's what Mike said in an interview with Guitar world magazine of december 1997, and I quote:
"On this record (Nimrod) I used Precision basses because my old Gibson G-3 didn't sound good with the new songs. I used an old '66 and '69;
they have a real warmth on the low end, and they have this London Calling midrange. For this record I knew we weren't going for the huge
guitar sound; this gave me more room to get in there. I needed to play a real punchy bass on the old records because the guitar frequencies were
everywhere and I needed to get right through the middle. I recorded Dookie with a P bass too, and you can hear the bass distort on "Welcome to Paradise."
I use Mesa/Boogie amps, MB 2000's. They're half-tube and half-solid state. I wanted to custom-make my own sound, and Mesa/Boogie was really willing
to work with me. They made me 6x10 cabinets, and we put those on top of an 1x18. I think I'm nailing it. I have an MB 2000 running each one of my
6x10 cabinets and then an MB 2000 running both of my 18's.