Vandersteen sub question

I looked at the excellent REL manual, I think the easiest route is to use a high quality Y splitter AFTER the M5-hp filter and run line level to the REL  using mode position 2 on the REL which bypasses internal REL filter. Run the Vandersteen side as you have been.
Factory is open today, give Richard or Nathan a call to confirm the Y splitter does not change M5 output.
Audioquest makes a decent quality RCa splitter

The amps in the Vandersteen sub have the inverse EQ curve to compensate for the M5-Hp rolloff, not sure how the REL is implementing the filter, but you don’t want to double filter the REL
i assume you are now trying to add a 3rd sub to approximate a swarm system ????