I have not heard the Audio Physics Luna subs, but have heard the REL's a number of times, and own a pair of Vandy 2Wq's. I think that the REL and Vandy subs both do a fine job. I have a personal preference for the Vandy 2Wq's when reproducing music, because I think they integrate extremely well with the main speakers (due to the hookup scheme that Vandy uses). The 2Wq's are very musical, due in large degree to the use of three 8"-inch drivers that are slot loaded into the room (slot loading smoothes the frequency response.) The Vandy subs are best suited for use with full-range speakers having flat frequency response down to about 40Hz.
Ultimately, however, your choice of the REL vs. the Vandy subs is a matter of taste, and I urge you to audition both in your home system if possible.
Ultimately, however, your choice of the REL vs. the Vandy subs is a matter of taste, and I urge you to audition both in your home system if possible.