velodyne dd12 or jl audio fathom?

I am preparing to purchase a set of B&W 802n's or d's. Will I require a subwoofer? The speakers will act as the front two channels in my home theater as well as my stereo. Is there a distinguishable difference to substantiate more money for the jl audio?
Yes the JL has impressive performance data - not perfect - some compression at 20 Hz but overall great value for money. Check out the HT Shack Subwoofer tests results I don't think you'll find a direct comparison with the DD12 but you can get the idea if you do a bit of research/reading.
A set of DD-12 measurements are here: Velodyne DD-12

You might also take a look at the SVS PB-13 Ultra SVS PB-13 Ultra
The SVS PB 13 looks awesome - for movies the ports can be left open and the 20Hz output is outstanding - no compression WOW, however the group delay is a wee bit high for my liking for music (may sound plodding or muddy in kick drum). No worries though - stuffing the ports (changing to sealed) will make it more musical (lowers group delay at the expense of higher cone travel and distortion - but I prefer sealed subs to get a tighter sound which is better on transients).

Actually any of these subs look good on how will they match to your speakers....that is tough question...only you can tell that.