The Xmax will not change at all if the available power has not changed. Boost or not.
You model for excursion control at max power and you could use 40dB of boost and there will be no damage. Bosso wouldn't bottom at max output even if he was just using one sub because he (or anyone) would have designed the system for excursion control at max power.
I don't use a HPF with my LLT's because I use them carefully. They need a HPF more than a sealed LMS-U, imo but I think people are just pushing theirs too hard where as I kind of tend to "baby" the things I own.
Yes, if I added boost down low (where I really don't need it) then my overall output would be limited because all my power would be used up down low. Only if I used a larger amp would I do any damage. Making my system "un-bottomable" from 10hz and above was a major design feature of my LLT's when I was designing them (and this is documented in my build thread and such conversations wrt my LLT's) so it's not really an issue. I wouldn't "run out of excursion" though unless I suddenly started using a much larger amp when I decided to add this boost.
Btw, MJ, you don't have to drop it. I'm not arguing with you at all, I'm just providing my POV to the situation. I get PM's about this stuff all the time and it helps to keep these discussions public.Edited by Scott Simonian - 6/12/12 at 1:00am