Exactly that's the doubt I have for long. If the speaker is good enough to play even 40hz like KEF R500, why not set the crossover to 60hz at least? Otherwise, won't we get directional sound from the sub? Won't it be bad for 2.1 music and even for surround music or movie? But in almost all forum posts, i see many recommend if you have a great sub like monolith ultra 15, set the crossover all speakers to 80hz or more.
I wish it was that simple. The sad fact is, the lower frequencies produced by different speakers in different positions in the same room will interact with each other and it is hard to know the overall effects because it depends on many factors. The lower the frequency, the more interaction between the speakers and the subwoofers.
In my setup, and I can post some graphs to show you, setting XO lower than 80 Hz could result in not so insignificant dips and bumps in the lower range up to 120 Hz or even higher. Remember, XO is not a brick wall, so if XO is 60 Hz, there could be significant output from the R500 down to 45 Hz or lower, again, depending on placement and room. The interaction may produce better (flatter) response, or worse (dips and bumps), that's even after running Audyssey. I found that with Audyssey, I could get very impressive response but I have to pick the right XO. Other Room EQ system such as Dirac Live may do better but I have not tried it yet.
Anyway, lower XO such as 60 hz may work for you in your specific setup in your room, or it may not, but you may still prefer the sound even with the dips and bumps that you don't see. Lots of people don't like flat bass response anyway.
So just try it, if XO 60 sounds better to you, great, if not then try 80, or 90 hz, as someone said, no right or wrong.
