Smaller subs are never an option.
Also, the insistence people have of putting the sub on the front wall or in a corner is baffling. I've experienced first hand what can happen when you decide that Audio Dogma is not law. My "best" corner that many people insisted I use for one of my subs turns bass into a loud and muddy mess of cr@p. Why would I blindly do that? "Because, you corner load your subs, dude, that's where they sound best!"
NO.
They can, I do not claim it is wrong, always, but no. Every room is different and we all have to make due with what we have.
First, do the crawl. Set aside an hour or two, unplug your other speakers, and play some good bass heavy music. then Crawl around your room with your ear near where the Sub Driver would be. Best way to learn to hear a difference: Move in and out of a corner a few times. Then as you move around the room, listen: do you find places where the bass gets boomy/muddy/dies? (avoid those), and when you find places where the bass is clean and clear, mark those. Perhaps you find the best spot is in a corner of your dining room, or where an end table is beside your couch. (I said before, keep your mind open.) You can find a solution if you are willing.
I am not a standmount/bookshelf fan. Yes I've been using such for almost 5 mos while I wait for my mains to be built. And a good pair can perform well. If you choose to shop for speakers, really shop! Salk's speakers mentioned above may be some of the best you can find. Yes they can get expensive, but if you consider the alternative and you just make lateral moves from lackluster speaker to ho-hum speaker, you won't ever be satisfied. The Song Tower and Song 3 could be some of the best speakers for the money! I would take them over ELAC, KEF, or B&W any day.
Happy hunting!