The answer depends on a few things. How capable are the two smaller subs. From personal listening experience, the 175 just does not reach low enough to compete with the 500. The 350 does.
The other is the characteristics of your room. In may case I get very little difference in response from my sub regardless of where it is places. I believe this to be because I have what turns out to be a square room at the subsonic level. My case is unusual though.
Blujays1: Spending Fred's money one bottle at a time, no two... Oh crap!