I actually bought both subwoofers. I bought a used JL Audio from a home theater shop here in town that went out of business and used it until the I got the Velodyne SMS-1 to set up the SVS Sub (which was really an impulse buy). Actually at one time in our family room we had a DD-15, a JL Audio F113, and the SVS PC13 Ultra, oh yeah, and the Magnepan's too. I didn't have any of them on isolation platforms, and they probably actually sounded different because the drivers of each of the subs affected the sound when each one was on due to proximity.
My wife made me choose one and get rid of the others. I loved the DD-15, I got if from a guy I work with who was moving, he ended up taking it back (same price) when I told him that I got the JL Audio (and SVS actually), I was going to keep that, liked it a lot until the SVS came and finally got set up with the SVS. The JL Audio now resides in my brothers home theater room, which is quite large, and it does an amazing job of moving air in there.
The great thing about Audiogon is being able to buy and test many pieces of equipment, with little cost, if bought correctly. I tried out a McIntosh C2200, and a ModWright 9.0 preamp after I bought my current Cary SLP98p, to make sure of which one I wanted.
I do the same thing with speakers, but I always seem to come back to the Maggies. I will prob break down and buy the 20.1's sooner or later, but I won't sell the 3.6's until I have the 20.1's in the system and decide if they are worth the extra money. As I am sure you know, the room, and mine is imperfect to be sure, has so much to do with the sound, that sometimes and upgraded component doesn't give any improvement in performance.