is it possible to connect a subwoofer to Crown XLS amps?

A picture worth 1000 words

Speaker level connection:

https://www.audiocircle.com/image.php?id=120343

Line level connection: 

https://community.klipsch.com/uploads/monthly_2017_01/5880223749318_ScreenShot2017-01-18at8_19_10PM....

My error. If the sub does not have speaker level inputs you have a problem. You do not have a preamp output on the amp. If your source has one you could use that as long as the volume is controlled at the source. It is an unusual way to do things. You might consider using a preamp. 

The Crown has an line level loop out of the 1/4" connectors.
You'll just need to buy RCA to 1/4" adapters and then run the line level out to you sub.

Done.