Subwoofer Crossover Setting/Directional Bass Freq.

My speakers are rated down to 28 Hz. I decided to experiment and hook up my sub woofer that I had in my home theater system to my music only system. The sub woofer (HSU VTF3 MK2) has a built in amp. I must be getting old because lugging a 70 pound box up the stairs took some concentration. At any rate the sub woofer has a crossover control. Should I set it to take over where the speakers drop off (28 Hz.)or should I set it to overlap the speakers say at 60 or so Hz.?

I have heard that at some point, bass frequencies become non directional. This might also come into play on which crossover frequency I use. At this point I may have to power the sub from another outlet/circuit and placement might have to be away from the main speakers. Does anyone know at what point/frequency bass becomes non-directional?

Lastly, I'm wondering if I really need a sub with my speakers that go down to 28 Hz. Initially after hooking the sub up to my music system, I'm not totally crazy about it. I may have to play with the settings a bit. It seems that the best setting is one that makes the bass very, very subtle. If the sub woofer bass is not set proportionately, it almost seems overpowering and takes away from the music. Maybe I would be better off without it.

I am new at all of this so if anyone has any suggestions or opinions on the matter, please free to pass them my way. Thanks in advance.