Your Marantz PM7000 has summed RCA preamplifier output for a subwoofer.
A simple one male two female RCA block (Y), a pair of inexpensive long RCA interconnects onto a subwoofers L/R RCA inputs would make a basic setup.
You could also add a form of DSP between the Marantz and the sub for far greater flexibility and control than any high level connection or basic crossover and phase adjustments.
When using the REL proprietary high level Neutrik/Speakon connection then simply turning down the gain to hide the subs introduction is instructed.
A simple one male two female RCA block (Y), a pair of inexpensive long RCA interconnects onto a subwoofers L/R RCA inputs would make a basic setup.
You could also add a form of DSP between the Marantz and the sub for far greater flexibility and control than any high level connection or basic crossover and phase adjustments.
When using the REL proprietary high level Neutrik/Speakon connection then simply turning down the gain to hide the subs introduction is instructed.
It’s hardly as seamless as using a low level connection from equalized signal processing preampfification which allows control of frequency and gain to better match the more relevant low level response character of the speakers rather than the amplifier.
Good luck with your choice.