Subs ideally should be set at wherever the main speaker’s bass output dies off...otherwise you can lose a portion of the music. I use 2 RELs as that’s all I seem to need, and they’re easily adjusted for level if a recording has too much or too little bass...I think Vandersteen was right as properly setup subs shouldn’t attract attention. If a single sub is too woofy, move it around until it's not.