what are your approximate levels on your current subs?
the reason i ask is that if they are on the low side of the range, adding two more subs could over power your room.
I assume that for music you do not want to increase the relative volume compared to the rest of the music, in which case you will want to decrease the level of the original subwoofers.
adding two more subs will give most of the room subwoofer impact at higher frequencies and more rumble headroom, not just the sweet spot or near wall boundaries.