Which sub - JL Audio Fathom or Velodyne DD?

I have two Vandersteen V2W subs for LFE, but they are showing their limitations. The V2W worked great in my previous smaller HT room (1400 cu. ft.), but even with two of them, the Vandersteen HT sub can't go low enough or play loud enough without strain in my current 3200+ cu. ft. treated room.

I've been doing some research, and I think I've narrowed down my choices to a pair of JL Audio Fathoms, or a pair of Velodyne Digital Drive subs. The JL Audio subs seem to have more output and perhaps a bit more low freq extension than the Velodynes, but the Velodynes have the digital room correction function. Which is the better route?

Lastly, should I get matching pairs (i.e. a pair of Fathom F113s, or a pair of DD-15s), or is there a benefit to getting a F113/F112 pair, or DD-15/DD-12 pair? On paper there seems to be benefit to mixing like this to get output advantages throughout the typical subwoofer frequency range. Does the theory match up with reality?
I know of no advantage to mixing driver sizes. What output advantages are you aware of?

We're talking about a very narrow frequency range 20Hz - 80Hz (assuming steep slope low pass filter). A 15" driver will handle that range easier than a 12".

My room is approx 3500 cu.ft. and a single M&K MPS-5310 handles that space easily. You say your space is treated, could it be that you have over treated the room?
Keep in mind the output of the F113 is similar to the D18. I would recommend two F113 which you can slave together, and they do come with room correction but not the pretty way via TV.

Buy the JLAudio and you will not be disappointed.. I almost would recommend just buying one to see how much bass it can produce... but if you can buy two subs it really fixes the bass room node problems and reduces the amount of correction anyway. The Velodyne correction is more difficult with two subs FYI.. download the manual and read about it..
There are only three or four subs that really work - despite all the claims - most people are listening to distortion (and satisfied with this situation as few have investigated performance data in their sub - see HT Shack website for many tests that show how badly most subs perform). JL F113 is one sub that does seem to work well. There are others but some are at $10,000 and aren't great value/performance or aesthetically pleasing for the home (such as the pro Genelec 7073 model)
I use two DD-18s in my 2ch system. Having the built in SMS and the ability to make adjustments in your listening chair is a real bonus. It's surprising how very minor changes in placement and config parameters will change the response curve. My wife walked in while I was doing some sweeps and she helped tame a slight peak - she makes a good bass trap.

I coudn't imagine doing this without a visual display, especially with two or more subs.
Hello Bob - my reading indicates that all other things being equal, a sub with a smaller driver can't go as low as a sub with a larger driver, but the smaller sub may have higher output in the mid-bass than a sub with more extension. Of course, all other things are rarely equal :-)

My room is not overtreated - it was custom-designed by a well-known acoustics consultant, and tweaked by another consultant. It's pretty much as good as it can get. I'm not surprised your M&K can handle your room - it's a much more capable sub than my Vandersteens. From the specs, it appears to have more output and better extension. The Vandersteens are nice subs, especially for the price, but they are not the last word in low bass or prodigious output by any stretch.
Curt at Velodyne tech support mentioned to me that the largest DD was faster & more authoritative than smaller models across the entire frequency range, due to minimal strain placed on the servo & plate amp by the short excursions of the larger cone.