The CXT is not an all out SPL sub, it is designed for very high output street beat and bass race use. I sounds equally as good as the SSX. I really do not care for the sound of BTL. The CSX is the SPL sub. I would go with the CXT hands down.
HDC3 and RE are not in the same league at all. I have seen the CXT take 2 CXT 6.1s for 30 seconds no problem, and the build is in a daily vehicle. Jeremy Camp who has the street beat 3 record. He is well into the 60s with 2 15" CXTs and 4 CXT 6.1s.