Mojo,
I don't think your receiver is limiting the lower end. I'm sure the DSP in the 600 cuts off below 17 Hz, and at 19 Hz for the 500. They have essentialy the same top end at 100 Hz, at least very close- take a look at both of the response graphs.
As Theo said, the 600 plays 2 dB louder and 2 Hz lower. Relatively very similar to each other in my opinion. I have been more than happy with the performance from the EP500 in my room. So I would tell you that you shouldn't feel like you had to make a big sacrifice if you go with the 500. But by all means, if you want the 600 and have space for it, then do it. It's your theater, so enjoy!
