I added a not-very-lovely image of the B.2500 Mixsequencer this evening. Yes, it looks like crap, but that's what I had to work with. At least I got the width right, even if some of the knobs are clipped in the present listing's pic. The reason for this is because I'm trying to sort out how to restore something akin to the functionality of the matrix switching. Here's what I came up with...
Looks daunting, donnit? The lower row has four different modules in repetition...outputs are paired with passive mults, audio inputs with fixed-level mixers, gate/triggers with diode ORs, and CVs with adders. Now, this isn't EXACTLY like what ARP originally came up with, because the matrix switches didn't have the extra circuitry that the 2 hp module sets below each have in this build. But it DOES restore some of the patching paradigm while streamlining interconnectivity and (conveniently) allowing for some patching methods that the original wasn't exactly capable of. And no...I wouldn't miss those fiddly slider switches if I were you. They were the real "weak link" in the original system.
In theory, it may be possible to hack the B.2600 modules so that the 2 hp "patch section" modules can directly connect to the module I/Os. This would clean things up a bit, avoiding little "jumper" cables between the Behringer points and the 2 hp patchers.
Also, when I first started futzing around with this, I used the Doepfer A-100 PMC6 as a "main case". It struck me that this is an EXCELLENT emulation of the original 2515 cab...and that would mean that you could pair it with the 84 hp A-100 cases to construct the 2508 "wings", like the original. One issue that could arise, though, has to do with that "hack" above...it would involve wiring between two rows, so most likely this would work best with some sort of keyed 2-conductor connections between the top and bottom for maintenance. But having a 2500 clone in an ATA-capable road case that's smaller and LIGHTER(!!!) than the original? Priceless.
FYI, this isn't necessarily an endorsement of Uli and his "business practices"...just an attempt at pushing out a new idea. I still think he acts like a bratty asshat, frankly...but if people are going to start diving into building ARP 2500 replicas, I may as well try and sort out this issue in advance.