Y ou can have speakers that go way down into the 20 hz range ( I have a pair 0f Walsh 5's rated down to 25HZ) and you still want experience the deep bass cross over at 40 as you will at 80hz. That is , if you have a real quality sub (I have the M&K ). Anything you take some of that low signal away from your sub and put it into your mains, or anywhere, thats less the sub has to work with and, its the speaker that is intended to handle these low signals. I STRONGLY believe in setting your crossovers to 80, no lower , regardless of the level of your speakers. I refer this to Home Theater applications only. Music is a totaling different story!