Originally Posted by jjackkrash
That's about where it is safe to run subs without localization, but localization can very based on the room and the amount of distortion your specific sub has. It is also where THX settled on setting the standard for crossovers a while back, and it kind of stuck. The truth is you probably out to start at 80 Hz and keep bumping up until you can localize, then bump back down a notch. You generally want the subs carrying as much of the load as they can carry without localization.
Good summary of the situation there
And just to add, I did a little experiment when I set up my most recent system a couple years ago with a pair of Dynaudio monitors and a Hsu sub. I wanted to test out a worst-case-scenario. So, I played some moderately loud bass-heavy music. Then I put my precious little head right between the sub and the nearest monitor. I set the crossover in the AVR to 80Hz. I didn't hear any bass coming from the sub. If just sounded "around me" and coming from the monitor. Good. Check.
Then, I moved the crossover to 60Hz. Again, no bass from sub's direction but a little less of the "enveloping" bass I had heard before at 80Hz. Ok. Nice but 80Hz was better with more "impact" and "roomfill." Time to check out the other side of 80Hz!
Next, I moved to a 90Hz crossover point (my AVR moves in 20Hz increments below 80Hz and in 10Hz increments above.) This was a real shocker! I could actually hear bass coming from sub! No way! I had always assumed that 80Hz was just arbitrary, but no, it's not at all. 80Hz was well-chosen by Lucasfilm/THX indeed!!
Granted I was mere feet from the sub and equidistant to the nearest monitor but there it was! At 90Hz I had to really concentrate, but I could clearly hear bass originating from the Hsu. Ok, so back to 80Hz and? Well, no more bass from the sub could be located! Amazing! I tried this again in a smaller room with the same result using a different Emotiva sealed sub and some Ascend monitors.
Of course, 80Hz might not be the best frequency for every AVR/speaker/room combo but it was for mine and it wasn't even close. So I would always recommend using 80Hz to start and then doing your own "head" experiment.