It's also a question that may soon be answered, although if Bob Crooks, the creator of the 25L15 was anything like Leo Fender, there were a lot of changes made along the road without a whole lot of documentation.
Here's what we know about it:
The 25L15 guitar amplifier was developed and built by Bob Crooks in the early 1950s. It's unusual in that it used 807 output tubes instead of the more commonly available 6L6 variants. As was common in those days, it was probably seen as good practice to put the power supply in the basement and the preamplifier stages upstairs. This arrangement could certainly cut down on crosstalk between the power supply and the preamplifier.
Crooks also used a JBL D-130 speaker, and it was probably the best choice for high volume, distortion free reproduction that could be had in those days.
Probably the mojo's in the preamp and tone circuitry and that's been the subject of much speculation. I've seen at least two schematics that are alleged to be 25L15 clones, but one of them is, according to Ted Weber the Hoosier Electronics Wizard, "not even close." Ted is contemplating releasing what he calls the Pennebaker 25, and if he does, then the cat will be well and truly out of the bag because he is a fellow whose veracity is unquestioned.
A fellow named Bill McKenna has also produced a schematic on DIY audio that you can obtain and read with jschem, a freeware utility.
Much of the speculation centers around the circuitry and its relationship to the Williamson circuit that was popular in the old days of high fidelity. That may well be so, although I cannot vouch for it and nobody who hasn't seen an original 25L15 circuit laid bare on the autopsy table or has an original schematic can.
We also know that production of the 25L15 by Crooks was very limited. There were maybe 100 or so built and they command astronomical prices on the rare occasions when an authentic one appears on the market. According to the list maintained by the good folks who are reissuing the 25L15, most of the originals are unaccounted for.
There are a number of people around who say they've reverse engineered the original 25L15 (as the people in California allege) or that they've reproduced it in some way, but they're bound to silence by a dark and bloody oath.
One thing's for sure. There're some rumblings that may well let this cat out of the bag. Then, and only then will we see where the mojo is hidden.