<font color='#000000'>Hi,
Adding more speakers to the same playback outputs in the same room causes many problems.
Will it make it "louder"?... Sure.
But, calibration of such a system should be left only to the pro's in large arenas trying to cover a wide area for large audiences.
1) If you mix different speakers, you will be able to pick out the better pair.... in a heartbeat.
2) Even if you add the same brand and model of speakers, they must be perfectly time aligned and arrayed so as not to cause serious phasing within your favorite sweetspot listening area.
My suggestion is this, if you are needing more "Umphh" from your system, don't buy more speakers, buy better speakers. I'm not sure how your 600 watt system is configured for distribution of that much power, but I can guarantee that better speakers will accomplish what you need without all the experimental hassles that would come from trying to add more speakers.
Hope this helps!
</font>