The interaction between the subs as mentioned above was my first thought. Another reason you might want to use bass management with a single sub is that your main channels can carry a lot of bass that is not sent to the LFE channel. Check out the waterfall plots from revenge of the sith...
Hope this helps
http://www.onsemi.com/pub/Collateral/MJ15022-D.PDF
and if the second one is a MJ15023
http://www.onsemi.com/pub/Collateral/MJ15023-D.PDF