Vandersteen sub question

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 ????

+1 Tomic.
Considering how the M5-HP is filtering frequencies above 80 or so hertz, if you added the REL via the speaker connections, then it would be receiving a filtered signal (-6 db? at the crossover point). I don't know if you would be able to counteract the effect of the crossover. And, I don't know how you could get an unfiltered signal without removing the M5-HP.

Please read the 2wq manual, it has a graph and explanation of what is going on.
https://www.vandersteen.com/media/files/Manuals/2wqmanual.pdf
Bob

I think the M5-HP,  when the crossover is set for the proper input impedance of the amp will be 80hz. with a 6db per octave rolloff for any outgoing signal of the amp. Correct? The Rel would have to have control of 80 hz on down, sub adjusted to signal of 80 hz. and output of sub adjusted for proper volume of the sub to blend with the main speakers of which are rolled off at 80hz. If this makes any sense, the Rel should work, No????

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

Thanks everyone for the input. I appreciate it.

@tomic601 
I forgot about a Y-splitter. Having no experience with REL subs, I didn't know there was way to avoid the internal crossover.
You da' Man...
Bob