Do I need a sub?

Would I benefit from even a small sub?

Energy rc-70s’
dual 6” woofs, bass is there, but would a sub remove the lowest freq, and give the small drivers a break, and sound better?

would it help,? I play loud once in a while, would the low freq removed from the main tower drivers make a better sound, I don’t think I would need much more than a 10 or 12” sub. 

Powered or unpowered?

In my listening time (30+ years), I've always used a single sub. The important thing is finding where in your room it sounds best. In my current home, I tried moving the sub two feet and it sounded horrible! Moved back to original location and it purred like a kitten! Just got to find the right place.
does it matter where a small small sub goes?

Usually. Since you have a variety of options, put the sub where you'll listen, then walk around the room until you hear the best bass. That's where to put your sub! :)
Any speaker with 6” woofer can use a subwoofer . Its whether or not you want more . And by taking the time to soul search and realize more of an audio ambition you have likely made the decision already . So do it. Powered 10” 
powered 12” is more 
powered 15” is quite lovely 
and so forth , you get the idea. 
I added to subs and I'm glad I did.  Rounds out the sound, better sound stage (helped floorstanders to disappear.
You don't need them, you want them!
My main speakers have built in powered subs that will go down to 20 hz and I still have an additional separate servo power sub.and it adds fill and energy to the room. Requires a little work to integrate, but worth it.