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

huh? can I not share the speaker level connections with either an REL sub or another sub that accepts high level?

will be used as an integrated amp to drive Magnepan LRS'
the amp actually has built dsp crossover functionality

Well, yes, but your comment above seems to indicate that you wanted to use the Crown to "power" the sub.  You didn't really explain what you were trying to do.

huh? can I not share the speaker level connections with either an REL sub or another sub that accepts high level?

Yes. You can. Sorry, don’t know why people are making this so hard for you. If the sub accepts line level and you have a line out then use line level. If no line out and the sub accepts speaker level then use speaker level. Simple.

So no worries. Hook it up. Enjoy.

sorry, I can see how my post might read funny
I've never done a speaker level connection from amp to sub
I've read of people with tube amps using different taps, and guys with a/b sections on stereo receivers using the second set of speaker terminals
a little confused on how to share the same set of terminals

should I use spade connections to share with the current bannana terminals?  never used anything but bannanas before so a little worried about screwing this up

Audiocanada, you sure can and you only need a passive sub as you have full crossover capabilities in the Crown. If the sub has a plate amp in it just remove the plate and identify the wires going to the driver. These are the ones you will connect to the amp. You should be able to identify  the + wire. If not don't worry about it as you can always reverse phase any time you want. I would solder 14 gauge speaker wire to these wires and cover the connection with heat shrink tubing. If there is no hole in the plate amp to lead the wires out I would drill a hole just large enough to fit the wire. Make a knot in the wire just were it will run through the hole so you can't pull it through. Run the wire through the hole then inject silicone caulking into the hole around the wire. That will prevent any leaks.
Now you are in business. Program your cross over and Rock! Forget about the speaker level connections just bypass the whole mess!