How is the Speakon connector wired for REL sub?

As I mentioned above your questions are answered here: https://rel.net/how-to-connect-my-rel/ find the session that talks about eliminating the hum.

2) Yes
3) No problem.

Try this to eliminate the hum, this should be in the video's I mentioned above, but maybe not.
Using the Neutrik Speakon plug connection as you stated that you are doing, now run a single RCA interconnect from the sub red line input to a red RCA input on your preamp. This should eliminate the hum.

Disregard my post directly above.
Three solutions to the hum problem:
https://rel.net/how-to-troubleshoot-and-solve-a-humming-issue/

The ground is lifted on both my RELs so no hum...one REL actually has a ground lift switch, and when lifted they both need to be lifted and that does the trick. I prefer the angled Speakon plugs...use em in pro gear also but many don't know the angled version exists...if the REL cables are thick (I use Canare Star Quad with the woven covers...looks great..) the angled plugs let you put the backside of a REL closer to the wall.

brf, thanks for that explanation. I have Googled for hours to find the wiring spec, as I need to make my own high level connection.