Set the crossover for what the sub manufacturer recommends; but, realize you will be re-adjusting it for each thing you play. This is the concern with a sub, they can be difficult to integrate successfully. My sub has a recommended 65Hz crossover when used with my JBL L100t3 mains. Since I do not have an adjustable crossover, what I've done is set the volume of sub to match my mains from the sub's amp. It's all I can do. For music I run my pre/pro in 2 channel stereo without sub output, except for house/dance music. I usually have a very satisfying listening experience. I do however have mains which do a really good job with bass as they were expected to do, since they were made at a time when subs were very rare, and HT, never mind stereo movies, were not yet on the scene. Today's HT speaker packages solve some crossover issues; but, adding a sub to a stereo system where a sub was not considered in releasing the mains, integration success could be limited; and, may require constant tweaking. Of course there are mains out there which pretty much need a sub but unless you buy a sub designed for those mains you're likely to need to constantly keep up that sort of system too. In other words, the system is really not a system.
