The Rel will not limit the signal to the mains. Just the sub. You would need an external active crossover like the JL CR-1.
I would get a pair of JL audio E112. They have built in crossovers. You send the preamp out to the sub. Then the sub out to your amp. Then just adjust the crossover as needed in a true high pass setup. This works very well.
I would not use subs without a high pass crossover. It will alway be poor.
I hear zero loss of sound quality (grain detail) running through the subs but my system modest, in crazy land anyway.
I would get a pair of JL audio E112. They have built in crossovers. You send the preamp out to the sub. Then the sub out to your amp. Then just adjust the crossover as needed in a true high pass setup. This works very well.
I would not use subs without a high pass crossover. It will alway be poor.
I hear zero loss of sound quality (grain detail) running through the subs but my system modest, in crazy land anyway.