Of course this is dependent on the (unconfirmed) assumption that the OP has a standard 8' ceiling:
16 X 13 is a bit better than 15 X 15,
but
16 X 8 is a bit worse than 15 X 8.
All in all, a rather small improvement.
I suggest that the OP use speakers with tight but limited bass response (sealed box?), perhaps with a diagonal room placement, but most importantly if buying new, available with a home trial and full cash refund. 30 Hz might be too much for such a small room. I would nix the idea of sub woofers, but multiple woofers might help. Room treatment and/or digital room correction should be considered.
16 X 13 is a bit better than 15 X 15,
but
16 X 8 is a bit worse than 15 X 8.
All in all, a rather small improvement.
I suggest that the OP use speakers with tight but limited bass response (sealed box?), perhaps with a diagonal room placement, but most importantly if buying new, available with a home trial and full cash refund. 30 Hz might be too much for such a small room. I would nix the idea of sub woofers, but multiple woofers might help. Room treatment and/or digital room correction should be considered.