Vandersteen sub question

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