The difference in a speaker's response near a boundary are two fold. An increase in bass output. An interference notch due to SBIR interaction -a form of comb filtering.
For a subwoofer the notch is not an issue because it will occur at frequencies higher than its crossover point. More at issue is boundary gain (3-9db depending on proximity to corners.) Ideally your single sub would be front ported or sealed, located midwall at a floor corner, and flush with face of wall with area around it closed in tight with drywall. This ensures the bump in output is even over its range and you can simply turn the bass down in your AVR.
You can search out papers from Harman if you want the explanation of room modes and placement for rectangular rooms. You are on the right track pulling your bookshelves into the room.
