Originally Posted by
ccotenj 
i would start with 80, and unless proven by measurements to be mo' bettah at 50, stay at 80... i would NOT do this strictly by ear, as you are already predisposed to "want" a certain outcome...

i would take a somewhat educated guess that 80 will be better, simply based on years of 'sperience setting up systems (not to mention having seen heaven knows how many graphs from others)...
as noted earlier... you need to think of your system as a whole, not as a
"am i getting the most out of one piece of it"...
for instance, my mains have measured (not spec'd) in room response well into the 30's... they are crossed at 80, for two very big reasons...
- crossing them there allows me to work with the two dominant modes in my room with the subs... with the combination of positioning/eq (primarily eq, including other tweaks), i make those modes "go away" in the end result... solving this issue can't be done by "ear", at least not by me, since in this region, peaks are very evident to my ears, but dips/nulls aren't... i am not alone in that, judging by ancdotal reports from many who report "i lost my bass when i eq'd my sub"... no, the user didn't "lose their bass", they lost the mountainous response curve, and likely eliminated a monster peak that they were identifying as "good strong bass"...
- regardless of the measured response (and if by some miracle the mains placement works well with the room modes, which virtually never happens), the mains cannot even dream of producing that 40-80hz octave with the authority that the subwoofers do... also, relieving the mains of producing that octave will allow them to perform "better"...
there's more, but those two reasons alone speak favorably of edging your xover points as high as they will go* vs. as low as they will go...
subwoofers have one job, and that's to produce 2 octaves (or in the case of some of us, 3 and a bit more

)... let them do it...
* assuming no localization, etc. and when it comes to localization, keep in mind that many times when people loclaize their sub, it's because of outside influences (rattles, etc.), not because of the frequencies coming from the sub...