I am using the speaker level inputs as in the ‘REL way’ of doing things, so I can’t do what Emil suggested- however, some other woofers do allow on the fly adjustments via a phone app. I could see this being especially useful where there is shared duty between HT and music. Luckily I’m just doing music with this sub.
As far as the -3 db point of my speakers, the manufacturer specifies 32 hz. This may be so in a perfect room with ideal placement, neither of which I have. When I use my sound check cd, I come in closer to 40 hz. So I basically am doing what Charles suggests. It is a front firing sub.
To further elaborate on the crossover point- the Hsu uses a 24db/octave slope- this makes it a pretty clean break at the crossover point. Other manufacturers may do the same- I don’t know.