My living room is 13 x 19 and I have an Arc + dual Subs + Era 300s setup and it sounds great. It isn’t overkill at all.

That is what I’d get for that room.

If you later feel the need to add a second Sub it is easy to do but you’d want to try repositioning the original a couple different places to make sure you couldn’t get the sound you need that way.

Thanks! Our couches are right against the back wall. Do you think it would sound okay to put the Era 300’s up higher than ear level, mounted on the wall? I’m thinking about a foot from the ceiling, and just turning the Eras upside down (our ceilings are a little lower than standard in our basement). I don’t have many options to place them. 
Also, where’s the best place to place the sub? Behind the couch or next to the entertainment center, below the TV?

Several folks here have reported acceptable results with the inverted 300s, I don’t think TruePlay has been updated to support that though. Usually not too big a problem.

Where to put the Sub is so room dependent that there is no good answer other than to try several places. I recommend a long extension cord so you can quickly A/B test multiple locations.

Normally in a corner gives the most low end boost but sound may be spotty.

Along a wall often gives less boost but more even sound.

Next to a doorway can reduce the low end. (boo!)

My open plan room works best with the Sub sitting at a 45 degree angle in a corner, so one opening is facing each side wall. Since it is behind the TV stand nobody can see it.

Folks have been happy with the Sub under their couch, seems to work really well there. If I laid it flat I’d put a swath of speaker grill cloth over the top opening to critter/kid proof it. I’ve done that here for my normally standing ones, we have far too many critters in Arizona that like to crawl into any opening they can find.