Velodyne DD15 - worth repairing the plate amp?

Goood to know. Looking forward (hopefully soon) to get plate amp back from EBC and do a comparison switch: Audessy with external class H amp vs built in Velodyne optimization and amp. 

What settings on the receiver do I need to choose when running Audessy for 5.1 (on the 5 and not the 0.1)?

I would just run Audessy normally then go into to manual setup and set the sub's xover higher than Velodyne's internal one. Adjust mains if possible to match the sub.

I need to read up on how Velodyne's software/hardware works. Does it have an equalizer? How does it measure the movement of the speaker cone or how does the feedback control work? Or what crossover frequency the Velodyne uses (fixed? or adjustable based on measurement with their mic of in room accustics)?

Have you ever used the video out on the Velodyne? The DD series uses the microphone to equalize the sub to your room. I believe the standard xover is set at 80. You can set the xover to whatever and for 5 different settings. You can also use the parametric EQ and manually adjust the output at different frequencies. But to do so you need to use the video out from the sub. There's lot's of settings you can access by using the video out. If you don't have a manual, download one.

https://velodyneacoustics.com/pdf/digitaldrive/DDManual.pdf