Preamp, Sub, and Magnepan connect

Possible ways are to get "Y" splitters and add them at the pre outs, with one pair going to the amp, and the other to the sub. Another (worse idea) is most subs have 'in' and 'out' RCAs to route the signal from the pre via the sub then from the sub to the amp.
Another is that some subs can accept power amp level signals, and you can biwire the sub into the amp output.
All of these ways leave 'something to be desired'.
Good luck

Is it possible to have very good technician put a 2nd set of pre outs on your preamp? That would make things less complicated for you.
It is a simple process.other than drilling the holes and soldering a new set of jacks. You will have one set for your amp and the other set for your subwoofer.

I don't like Y connectors personally but they are functional and work as well as a pre-out. Foillow the instructions for using them from Elizabeth above.
Make sure your interconnects are a snug fit to prevent it from coming loose which will cause "@%^$#@" (mayhem).
The JL is a powered subwoofer and you don't have to do anything but hook it to a preamps output and make adjustments on the volume and crossover.

buy a Rel sub it will take both the speaker level input and the LFE from a processor/reciever so you can run both systems.

You can split the output from your preamp, I did that with some DH Labs splitters that I got from Upscale Audio and I never noticed any loss in quality.

good luck

thank you for your immediate response, This chat forum is great. One followup queston to Macdadtexas, Are you suggesting REL instead of JL audio?