I agree that from what I have heard with the Bart MM pickup it definitely has the "Bartolini signature" in the tone, and is a little more subdued in the upper mids/treble than the classic MM sound.I'll pop in here as another data point. I had a nice Lakland 55-76 (their short lived exact MM placement model), that came stock with the Bart MM pickup. While it sounded good, it really didn't cop the more aggressive MM thing very well. It had that strong, warm, low mid present Bart sound that would be very comfortable for those who like the standard dual Bart pickup Roscoe models. Kind of like a 'MM on satin' sort of vibe... smoother ping up top, rounder upper bass response down low.
I had the Seymour MM installed to try to get closer to that MM wide, deep, sizzly sort of tone, and, to my ear, it went a little bit too much in that direction. Massively brighter than the Bart MM (lots of upper midrange), and much wider and less punchy down low. So, too much of a good thing for me with that one. It did the MM series thing VERY well, but to my ear, fell short of the classic MM parallel tone (and of course, without the dummy hum cancelling coil, the single coil setting was unusable. If I remember correctly, the Bart was basically two HC pickups in a single case, which kept it hum cancelling when coil tapped... been a while though. The Seymour was definitely 'single coil' when tapped).
To date, the best sounding MM replacement pickup I've heard is the Nordstrand classic MM model. However, I believe it does not have a series/parallel switch option (I always used my MM in the more classic parallel setting, but for those who dig the grittier, punchier series setting, I don't think it is doable).
So, to my ear, the MM Bart will do the best job of 'flavoring' the Roscoe stock Bart tone toward a more MM thing. The Seymour would take it to a totally different place, and with some pretty extensive EQ, it sounds quite good to me.
Edit: These comparisons were made with the original 3 band Bart preamp used by Lakland, which I believe was a stock preamp.
I am guessing the 55-76 was an ash body? Also worth noting is that the Duncan comes in both Alnico and Ceramic versions - what you may have had was the ceramic, the way you describe the tone. Not that any of these things are absolutes, but compared head to head the Duncan SMB-5A (Alnico) and the Nordstrand actually sound very close with the Duncan having a bit more edge and the Nord a bit smoother.
What really sold me on the combo was Jon's tone with the Duncan (ceramic) in a mahogany body Roscoe, the first CS5+M http://www.talkbass.com/forum/f153/d049-monkeyman-855526/
A great example of this can be heard in the last track "Seven Promises" on his band's album: [Invalid or Expired Link Removed] (buy the album, it's great!) More clips here: http://youtu.be/F7BXL7XPXYE and here: http://youtu.be/Ll9rP7E81ng
Compared to the Monkeyman, my bass has a few differences that will impact tone a bit:
-Alnico vs. Ceramic
-East MM preamp vs. Aguilar OBP-1
-maple/purpleheart/maple neck vs. all maple
-purpleheart fingerboard vs. birdseye maple
-thicker maple top vs. thin monkeypod top (impact may be minimal)
When it arrives in the next couple weeks I'll weigh in with my thoughts



