Got the PE 250 (plate amp) yesterday. I see how you hook it up to the sub, read manual to see how you hook it up the the receiver. But I see this crossover only going down to 40hz. And I heard Phil say disable the crossover on the amp. But I don't see how you can do that. Unless your supposed to modify the crossover like the book is saying. "Locate R26 and R27 on the pre-amp board. Using a low watage soldering iron and desolding braid, remove the two resistors." Is this what I have to do to get to 20Hz?