Here's about everything you'd ever want to know about the Ampeg Devil Basses: love of Ampeg and other Scroll Basses
And Part 2 is just getting going: love of Ampeg and other Scroll Basses, Part 2
And the page from my own web site: http://www.xstrange.com/asb1.html
At the moment, I have five of them here in my shop. Two of them I have in my own personal collection, and three are customers' instruments here for restoration. I was working on one of them this afternoon, scraping the paint off the neck and preparing it for structural repairs.
Yes, they are strange old basses with a lot of problems in the design and construction. The "mystery" pickup is a mechanical-percussive diaphragm pickup. The bridge is mounted on a drum-like steel diaphragm, and a magnetic coil detects the movement of the diaphragm, not the strings. It creates a unique sound with a heavy pulse on the attack curve. It'll never sound like a normal modern bass. You may love it or hate it.