Max extension will gives you a lower frequency response than max output which will sound slightly louder. It really depend on your room. I run mine in max extension because that is what I got it for. My mains go down to about 35 hz and when I want the room to shake the sub takes over. The volume setting is also purely subjective. If you do the sub crawl correctly and find the sweet spot you will find that you can turn it up somewhat and not get the overpowering subshake effect. Something I did which was really helpful is to use some low frequency test tones and move around the house to see what is rattling about. Most of the time a little repositioning of isolation goes a long way to making the wife happy. I actually rattled a pane of glass out of a window during the Heath Ledger Batman movie. I think what Sdurani was saying is that since the LFE is pretty directionless, you could place the sub behind the couch and not do the subcrawl and it won't matter a lot. If you are closer to the sub than the peak node of the room you won't hear a big delta but something in the room might feel it.