The reason was simply that we're using 4715's with 1800w per cab and I don't trust they can handle that low (so yes, technically not enough rig). They're rated -10db at 40hz. I may have been wrong, but it's a moot point now as today I am buying back my old 4719's. They're -10 point is 25hz. I run them with only the 30hz filter on the amp. I know from past experience with them that engaging the 40hz on the xover made an audible/tactile loss of low frequencies. I don't hear/feel the difference doing that with the 15's. All is right with the world again. Well, with the possible exception of my bank account. But hey, it's only money right? They print more every day.