Well, my experience with the L2500 has been that it's down right "rude". It's snarly and aggressive. It can be kinda like a "Pbass", or kinda like a "Music Man", but it always has this sort of "lo-fi" thing about it. It's got a lot of bottom and it's bright, but everything is kinda in the mid-range. Lots of low mids and high mids. The treble doesn't actually extend very far up - it never sounds "open" like a jazz bass w/single coils, and the low end is huge, but it's mostly low mids rather than deep bass.
I felt like the M-series sort of has more polite, polished mids. The bass and treble extend farther, while the mids are more tame. It still has a certain G&L aggressiveness to it, but the EQ opens up a lot of options, and the rewound pickups are more even sounding. Now I was easily able to get a GREAT slap tone and a really burpy finger-style tone, as well as a warm Pbass type sound, but also a "rock" tone that rivaled my former Spector Euro5 w/EMGs. The treble was just much cleaner and more open than the L-series pickups, and the bass was bigger, with the mid range dipped out of the way. The B-string was every bit as good as my Spector's. The M-series has a certain punch that my Spector had. You can really feel each note hit you in the chest.
In some ways, The M-series is almost like running an L-series through an SWR Aural Enhancer - extends the bass and treble, tames the mids, only the treble is even more open sounding. I can sorta get close with an ACG filter preamp cutting everything above about 100Hz, leaving everything below that alone, and boosting the highs from about 3k up.
That's the best way I can describe it. I'm not sure how useful it is, and I'm not sure if anyone else will agree with my assessment. FWIW, I ran an L-2500 in single-coil mode for a couple years. I also tried various EQ and wiring schemes and never could get it to open up like my Jazz basses and Spector. I still have a fretless L2500.
Hope that helps.
