I love my M2500, it is the best playing and sounding 5 string I've ever owned. It's a lot bang for your buck.
The M and L series are the same structurally, it's the pickups and electronics which differ. The M series features a pretty standard volume, pan, bass, middle, treble setup which operates active only (18 Volt). It lacks the active/passive option and switches found in the L series. The M series pickups are not quite as hot as the L series.
Having played a Stingray for several years the preamp was very intuitive. I can dial in Jazz/Precision/Stingray tones with ease. It's a great all around bass and it's versatility is hard to beat. The M still retains the aggressive tone of the L, but you can mellow it out. I have read where the L can be a little tough to tame. In my opinion the M solves this issue without taking anything away from the legendary G&L tone.
"if you never set goals you can't fail."
G&L Club Member #527, Stingray Club #475