Sub crossover setting....

Seems to be 3 camps that offer differing suggestions in regards to sub is to set the sub far below a speakers cut-off point(my REL dealer)...another is to set the sub at exactly the frequency a speaker rolls of(for instance 40hz for a full range speaker)...and yet another that claims the sub has to be an octave above the speakers cut-off point...for instance 40hz would equate to a 80hz crossover setting on the sub...I believe Vandersteen is a proponet of this set-up...whatz up!
Invest in a Rat shack analog sound level meter and one of Rives test cd's. They have the adjustments for the meter in their instructions. That is the best way to set the X-over point. Happy Listening! John
The Rat Shack is quite linear throughout the bass region, so other test CDs with bass warbles will work too. You're predominantly interested in flattening the 40, 50, 63, 80, 100, 125, 160 and 200Hz points into as flat a response as possible. Above 1k the meter gets crazily inaccurate, but VERY reproducible if you get the older analog one. Have fun, and realize that response will vary A LOT depending upon where you put the mic/meter, so listening position head-level is a good starting place.
I don't use any sound level meters (although I own one) or rules of thumb, if you have adjustable crossover on sub just start adjusting while a song with steady bass beat is will know when crossover is set too high because of pronounced bass hump/fatness which is easy to hear, just adjust crossover level down till hump is gone and sound seems well integrated/seamless to sub.

Best to have a friend who can adjust crossover while you sit in listening position but the effect is quite obvious even when standing over sub adjusting crossover by yourself.

For me best crossover level to sub is often very close to main speakers rated on axis +/- 1.5db lowest bass level.....of course where you put sub is a whole other story.
I might add there are much more scientific ways of doing this using sound level meters and test CDs as some have mentioned above, but the quick method I just described works very good for me.

Once set I keep the crossover point constant for a given pair of main speakers, but I do adjust the subs volume level a bit depending what type of music I am playing and what type of bass I want to hear.