I've said before that the one bass that I miss more than just about any other is the Fender MM IV that I sold to a lad at school about 3 or 4 years ago.
Having recently acquired a mint example at a good price and having bought a Sadowsky onboard pre with VTC from Chris (Beedster) the way ahead became clear. The MM pre isn't noted for it's desirability and common upgrades are the Marcus Retro in the UK and across the pond, the Sadowsky pre.
I did the swap yesterday and despite a fair bit of soldering it worked straight away sounded great. The only minor issue was that the blend worked in reverse. I decided to sort this, an easy swap of two wires on a pot. I brought the bass to the soldering table and plugged the lead in while it was still disassembled to check it out. Sadly I had a really bad buzzing coming from the bass that changed when I touched the pots. I then spent nearly two hours taking it apart, putting it together, checking all my connections, double checking them, continuity testing, putting in extra ground wires etc.
The in desperation I turned off my Maplin adjustable temperature soldering iron. Problem solved
Bass sounds great too, very pleased with the upgrade.