The "headphone jack" is for the stock pickup. The piezo is connected into the original harness via the jack and this is re-used with the new pre. The piezo lead terminates with a male 1/8 plug and using the old jack keeps it clean. You will lose the actual headphone jack with the replacement pre, but a headphone amp is easy enough to plug into and pretty cheap to get separately.
The power is now activated by plugging into the 1/4 output jack, removing one point of potential failure in the switch on the stock setup.
I've got no complaints with the pre and it has been flawless through years of gigs. The cheap knobs I picked up have been fine too. One of these days when I am feeling ambitious I'll replace the jack plates so they match. I've been using it exactly is it is in the photos I posted when I installed it and the 3 ply volume plate doesn't bother me at all when playing as nobody can see it.
Aha! Thanks for clearing up my question about the headphone jack. That makes perfect sense.
Also, for verifying the 1/4 output jack is the on/off when you plug in.
It looks to be a very straightforward swap and installation. With your pics and explanations I'm confident I'll be able to modify mine in the same way. I'm looking forward to
trying this out.
Thank you, too, for the real world updates about still using this setup and that it's bee working flawlessly. That's a huge endorsement.