They are very musical subs.
If I were you I would look for one use - they can be found for $500 or possibly less. I myself have one for sale, but want Boston area pickup. You must have a separate pre & amp to utilize the in line crossover.
I replace my Vandy with a BagEnd Infrasub 18, which can be found used for $800. I have never heard bass as tuneful as the BagEnd, for me, after about 5 minutes, all my other subs went out of the audio room.
Both of these options will have less SPL's than the Velo, so perhaps buy two and use them in stereo. That really makes a big performance leap again.
I have heard nothing but positive comments on the REL, and on the ACI Titan II, but have heard neither.
Dan G.