I would strongly recommend the use of 2 subs. One sub is often “impossible” to locate when you divide low enough until you buy a second one. Then you will notice small differences in the soundstage and some tactile improvement. There are white papers from several sources like JBL who explain the advantages of using 2 or more subs. I use 2 Paradigm Servo-15 in a 5.0 setup up to 120Hz. Depending on sub and the number of them you have the possibility to use higher cut off frequency. I see some important advantages of using higher cut off frequencies;
1. Less power is sent to the L-R with less distortion generated in L-R speakers.
2. 80Hz-120Hz let some off the lower overtones still to be sent from the same speaker
3. If you use your setup for cinema as well a cutoff frequency of around 80Hz is recommended according to THX.
4. Group delay at crossover lower than 80Hz will cause increasing problem with the speakers getting out of time.

Personal preferences, room acoustics and different equipment will of course give different results but this according to my practical tests and measurements and according to many companies findings and standards.

I found one web page, which I found interesting and I will try out different filter settings and measure to see if further alignments between the different speakers would improve the soundstage even more. I`m happy today but as always you like to have even more fun when listening to music.
http://freerider.dyndns.org/anlage/LeCleach.htm

Regards
Harry