I have an M-Audio BX5/SBX set-up myself, and that's where my sub is--centered, just below my mixing desk, away from corners. Sounds great, works fine for me. Many factors affect the overall sound, of course, depending on the level of quality you need. The pro folks here will wisely suggest room treatment for higher applications, but I haven't found that necessary for my basic needs. Either way, your BX5As will likely surprise you (in a good way) when you add a sub. Just take plenty of time to experiment with the exact placement and mains/sub balance.
Best,
J.