Do I need a sub?

The answer to “Do I need a sub?” is always yes. Even if you already have one, or two, or three, or more 😀 Multiple subwoofers is the path to high end bass and high end audio in general. Yes, if you high pass your mains they will play louder and clearer because it limits cone excursion on the lowest notes and hands that duty to the subwoofer which is designed specifically for that task. That’s what I do in my system and it sounds great. I’ve got two subs and my end goal is at least four. Not for loudness (although that’s fun sometimes) but for bass even-ness, extension and clarity. When you get a sub, invest in a MiniDSP module or something with DSP capabilities. It lets you precisely control the crossover and crossover slope, apply notch filters to help minimize room resonances, and do other things like apply Linkwitz transforms which can extend the bass lower as long as you have the amplifier watts and available cone excursion (for sealed subwoofers anyway, not sure if that works with ported). DSP and subwoofers should be married to each other in high end audio. Best of luck to you.

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!