The FV18 runs about 9db hotter than my F15HP due to the larger driver and ported design. You have two of the FV18s so I can only imagine how well they would fill a sanctuary right down to 16hz. Rythmik Audio makes phenomenal subs and there's nothing imaginary about their subsonic capabilities.
So I haven't heard the FV18 but it's a product I would buy on reputation alone and on my own experience with the F15HP.