I know I don't post on these forums much, I'm more of the sit-back-and-listen-to-people-who-know-more-than-me kind of person. Which is why I'm posting this.
I own a Fender Marcus Miller IV. It's great, I love the feel of the neck, the bright tone of the maple neck, the placing of the bridge pickup to get that certain growl, etc, etc...
But there are two things I'm not very satisfied with. One is the pickups, they hum quite a lot. I pretty much have my heart set on a pair of the humbucking, Nordstrand NJ4SE's, so thats not the dilemma. I'm not very satisfied with the preamp. I'm looking for a new (better-sounding) preamp that requires no routing of the body or drilling whatsoever. A direct replacement. I don't want to spend the some $350 on the John East one.
I play in a 60's/70's rock/blues/funk cover band, though we do cover some newer things, if that helps. The search function didn't help me much, but if this has already been covered, if you could direct me to the thread, I'd be glad to delete this one. Thanks.

east is the direct replacement...

My vote goes to Mr East.

You might look at the Sadowsky onboard pre. IIRC, Miller's actual bass has one. I have the MMV, and in direct comparison, the MMV sounds quite a bit different than my Sadowsky Outboard Pre DI, which is supposed to have the same circuitry as the onboard. Personally, I would look into an Aguilar OBP3. I think this bass would benefit from having a mid control.

There was a thread recently discussing this, and Marcus Miller has a Bartolini pre in his bass, Roger only installed it. It even mentions this on Marcus' website.

The 5 string miller has a fender pre, that they designed with Miller to get a similar sound to that of his 4 string, but that works better with a 5. Check out the FAQ on his website for all the info...

Aside from that, I am no help. Maybe see what happens with the new pups in it first??

the preamp in the MM5 sounds "NOTHING like the preamp in the IV,hell it's not even close.