| I recently had a Climax in my possession and you're right, there's not a ton of info out there. I can't really speak to the differences in preamps because I don't own any other active basses (I honestly thought it was the same as the L1500, but could easily be wrong), but here's what I can tell you:
The Climax came standard with the #6 neck (1.5" nut, 7.5" radius) whereas the L1500 comes with the #7 (1.75" / 12"), although others can be custom ordered. Assuming you're correct that the preamp is different, the controls are, in fact, the same (1 toggle for active/passive and another for series/parallel).
The only other difference that I know if is simply cosmetic. The Climax has sharper edges all around than the G&L basses of today. It also has an interesting "scoop" in the back where the neck plate sits. Presumably this was done to reduce weight.
The model was produced from 1992-1996, so it's one of the earliest post-Leo designs. Word has it the design was a tip of the hat to Leo's Stingray design, possibly in tribute to the late genius (who died in early '91). No idea how many were produced, but I think it's safe to say that aside from limited edition and custom shop instruments, it's one of the rarer birds.
As far as construction and quality go, this bass is every bit as nice as the G&Ls of today (CNC and Plek accuracy aside, of course).
Frankly, the only basses G&L ever made that might not be considered "top of the line" were the SB series (which stands for "student bass"), but that's really misleading because these basses reflect every bit of the trademark G&L quality. Originally, the SB's were a bit less expensive due primarily to limited options and the relatively simple (compared to the L2K) electronics. Also, the first generation basses had slab (non-contoured) bodies, which presumably saved some coin in the construction department.
As the 2nd generation SB-1 and SB-2 grew in popularity, G&L no longer marketed these as student basses. Today's SB-2 can be every bit as flashy and nearly as expensive as models with more complicated electronics. I say this as someone who owns two of these puppies.
Sorry for the long-winded explanation and the SB digression. Hope this answers some questions. | 
