Dual 500s may give you more homogeneous bass but they will not offer you a lower frequency response. I don't know what you might be missing with a 500 over a 600. I know Sirquack had a 500 for his 8,000 cubic foot theatre and recently upgraded to a 600 and he's been saying that they are very close. And I agree with what JohnK said about the rears. Good decision on floor-standers BTW.