Crawl it out!
Without having to play around with measurements, put it at your LP and hook it up... disable your Speakers... and play some good bass-heavy music (that you know well) through the Subwoofer alone.
The Sub Crawl is crude, but useful and can teach you a lot about how Low Frequencies behave in your room.
It is not fool-proof, but nothing really is in life.
Yet, you can get a very good approximation of what the bass response will sound like. Fine tuning may still be required.
The first time I did this, it took me about an hour to crawl out my room. I played with test tones, pink noise different music. I settled on music... test tones and pink noise to me weren't revealing enough.
As far as what to listen for, you want to find a location that hopefully sounds clean. Boomy and muddy are bad, likewise, so would a dead spot.
When I started, I moved along one wall, and as I got closer to a corner, I started hearing the differences that can happen. In my room, the corners get very sloppy sounding (boomy, muddy). My cleanest bass was on the side walls. On my back wall, I had a serious null/dead spot: the bass just disappeared.
Every room is different. So even the most experienced are hard pressed to look at a space and say, "there!"
In fact, you may find that the best place is nearfield right behind the couch.
Just keep your mind and ears open when you do the crawl. Don't rule out a spot before you listen.
Do the Subwoofer Crawl. It's the easiest way to find the proper placement location for your subwoofer to get the best performance and sound.
www.audioholics.com
(I agree with Pogre... you should be looking at your XO in the 80-100 range. Most people I've seen discuss this agree that this is where they get the best performance out of both the Subs and Speakers.)