Attaching a Rel T9i to a Krell 402e.

@lak has given you what you need to know. The short of it is to use the high input with the Neutrik connector. Each sub hooked to 1 channel R & L. 

Rel is usually good about trouble shooting but just in case I'll tell you that it is recommended to remove one of the hot wires IF you volume adjustment cannot go over half volume. This was the case with mine.

Will this help?
https://rel.net/blog/2018-01-04/how-to/how-to-connect-my-rel/?utm_campaign=2020-12-25_broadcast_cont...
Or maybe select one from their series of how to?