I have a 2013 USA M-2500, and I think the B string is great. It might not match a good 35" scale B, but it does well above the 5th fret, and it's much better than a 2008 Fender USA Jazz five I owned previously. Part of this, I think, is the extra-rigid quartersawn neck on my M-2500, so if you do a special order I recommend that option. The rigid neck and six-bolt neck joint provide a very solid platform, with lots of clarity and sustain.
To fully understand the differences between the L-2500 and M-2500, watch Ed Friedland's video on YouTube comparing them. He goes into serious depth. I felt comfortable going with the M-2500 largely because of his videos. I had never tried one and normally don't like active-only basses much at all, but I absolutely love the M. It's a phenomenal bass.
Here's one:
https://www.youtube.com/watch?v=T95hIdjrzwc
Another:
https://www.youtube.com/watch?v=Rpde-ZSNeW4 (Focused on M-2500.)
Basically, the L-2500 has the original MFD humbuckers, three-way pickup selector switch, series/parallel switching for the pickups, passive cut-only bass and treble controls, and active/passive operation. Active mode just switches the output impedence -- the tone controls remain passive. Then there's active with treble boost. The M-2500 has a slightly less powerful version of these pickups that are wired only in series, and then has a conventional blend control and three-band active EQ. As you'll see in Ed's video, the two have lots in common sonically, but each does things the other can't. For example, I'd love having the L's passive operation and more powerful pickups, but the M's blend control is extremely useful.
The L-1505 and Kiloton have the more powerful MFD humbucker of the L-2500, which is placed in the so-called "sweet spot," i.e., slightly toward the neck relative to the bridge pickups on the L-2500 and M-2500. (Similar to a StingRay.) This can produce a very aggressive tone and this pickup placement is great for a B string, but it doesn't always provide the fatness I typically want. (I used to own an L-1500, so I have experience with this.) Of the two, I think the Kiloton 5 is cooler, because it dispenses with the preamp and has series/parallel/single coil switching for the pickup.
I'd like to try a Kiloton 5, but I prefer more of a thick "neck" tone to a cutting "bridge" tone, and I wasn't crazy about the Kiloton 4 when I tested one. Besides, I can nearly duplicate my old L-1500's tone by panning my M-2500 slightly toward the bridge pickup.
I think empress wood is a great option. I've been on the lookout for an empress L-2500 myself, but they've all had rosewood boards and I want maple, and I also want a quartersawn neck which isn't easy to find.
