Subwoofer insight.

A simple 6db/oct XO HiPass is easy to make.

See http://www.ielogical.com/assets/Audio/PassiveXO1.png for a picture 
The circuit is just a Y with the appropriate cap in the main amp line.

Fc = 1/(2 π R C) where R is Ω and C is in F. 10nF is 0.000,000,010F
R is amp input impedance
See  http://sim.okawa-denshi.jp/en/CRhikeisan.htm

As a general rule for small 2 ways, ½ to an octave above the claimed -3db point is a good rule of thumb. e.g. for 60Hz speaker, 85 to 120Hz. 85Hz is not exactly ½ octave, but close enough.

Use good caps. Teflon or Polypropylene or Polystyrene.
Avoid mylar and worse.

I highly recommend REL and have owned several over the years. Simply visit their website and they have a page where you can enter the data of your room size and speakers and they will match you with the perfect sub. You absolutely will benefit from the addition of the sub in your system.  

Simply visit their website and they have a page where you can enter the data of your room size and speakers and they will match you with the perfect sub

Nonsense!
Limited speaker brands, model choices and room parameters. An utter waste of time!

Some of the most egregious sonic travesties experienced have included REL subs in dealer show rooms.

20 years ago REL were tops and they still make some very good subs today.
Others are price point pathetics.

I started my REL experience a while back with a used (200 bucks) Q150e 10" 150 watt little gem that works perfectly. I'm big on the "high level" Speakon connection as long runs of RCAs is just silly to me...balanced if available would be fine as is the wireless feature on new subs, but otherwise I simply made my REL cables from Canare Star Quad and it works swimmingly. My second used REL came along a couple of years later as a Q108MKII 100 watt 8". Also 200 bucks, also perfect. I'm in the "2 subs or more" crowd as it just makes bass seem more balanced and effortless...however, if one sub is all you can do, do it. Running the main speakers full range also sounds best to me, and since they're very efficient my main amp doesn't need any help with crossovers limiting its bass...I dial in the best sound from the main speakers and then roll in the subs to where they just sound right. Note my room sounds like a room, the room tuning consists of furniture, a rug, and a high sloping ceiling. No deleterious reflections or bass boom issues in my sweet spot. I turn the RELs up and down a little from time to time relative to the recording and very rarely use a Schiit Loki EQ...works for me.

Probably not what you want to hear but I think you should get properly sized stereo speakers for your room. Start there. It sounds like your current speakers are not up to the scale of your room.

Next, find the right location for those speaker and then treat the room. I don’t mean use crazy amounts of acoustical treatment. You can do a lot with very simple choices you make with existing furniture/art placement. One compromise my wife makes is we have no glazed picture frames in the living room. All the art on the wall is sculptural or framed oil paintings...some framed textile art. We have a 9x12 Persian rug and carefully placed furnishings.
It does not look like a listening room and isn’t perfect. But I get slamming bass and holographic soundstage in my listening position. No need for a sub.

I own two subs that are put away. Same issue: constant adjusting of the bass levels. In fact the imaging improved without them. My stereo amp doesn’t need to compete with the powered subwoofer’s transformer.
When you dial the main speakers in...then dial the subs into the mix.
I can’t imagine that a single 10-12” sub wouldn’t suffice for a room of your size. But I tend want subwoofers to just ‘fill in’ what’s missing. Some people like room shaking bass.