If the shape can be anything, why not make it 17 Hz? For a 15" driver most people expect far more that 20 Hz at 6 Db.
My guess, considering we're talking about REL, is that they wanted to avoid aggressive response-shaping to minimise their use of filters.
Specifically, -6dB at 20 Hz is, IIRC, THX standard, where it's expected the room will help linearise the response flat. In addition any excessive shaping would result in a loss of headroom (if they wanted a -6dB point of 15 you'd need to sacrifice a heap of output elsewhere, or have dynamic limiters, which they apparently didn't want to employ to avoid the response shifting with SPL)
Looking at
@shadyJ's measurements, lifting 15 Hz by 10 dB to extend the output would need a heap of extra filters and a more aggressive protection-scheme, sacrificing headroom and trying to force the driver to do something it doesn't do very well to begin with.