Best Way to Split Amplifier Output Frequencies to Main Speakers and Subwoofer

Is your sub powered?  Many subs have a crossover built in.  It might be cheaper for you to buy a new sub.  Part-express sells subwoofer crossover in a variety of price ranges.

DBX makes a nice crossover.

Does your receiver have preamp outputs and power amp inputs? Make and model would be helpful.

Rel has a  high level input. IOW you just connect the sub to the speaker terminals on the back of the receiver. Then adjust accordingly. They integrate well and make the mains sound better. The trick is to keep the volume & crossover as low as possible so as to blend with the mains

Yes, but the REL doesn't have a crossover to keep bass from the mains.  Keeping the amp from having to replicate low bass may help with headroom???

Pass, First Watt, Bryston, Marchand. Both low pass and high pass filters, variable frequencies and slopes.

+1 elevick. And running through low-level inputs give you deeper bass. No answer from OP on separable preamp outputs and power amp inputs. I'm stopping.

Are your mains ported?  Seal them.

It won't be as good as a true crossover, but it will extend the dynamic range and help integrate with the sub. 

Regards,

Erik