I bit a deal from woot.com for an Athena as-p4100 which sounds great after 2 days, 10x better than the stock sub from my Onkyo htib.

HOWEVER, my buddy's selling an older Velodyne VRP-10 for about the same price. If I can unload the Athena, should I go for the Velo?

I think I would stay with the Athena. IMHO the VRP-10 is underpowered and doesn't have much authority in the lower range. IIRC, isn't the VRP-10 only spec'd to 35 hz @ -3 dB? I think the Athena can do better than that.