Why not send Velodyne an email with your question? BTW, I've wondered the same thing. I spent a little time comparing the two, but nothing striking jumped out at me.
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DD has the OSD with a 8 band EQ that can be adjusted manually. The SPL only has a 6 band auto EQ. You can't adjust it manually.
The DD also has a larger amp at 1,250 RMS, dual 3" coils, and servo control monitoring the voice coil 16,000 times a second. Much lower distortion.
Volume wise there won't a huge difference, but the DD plays much lower with lower distortion. The two subs aren't in the same league.