In my opinion there was a noticable change from the 500 to the 600. I think a lot has to do with the overall size of the cabinet, the LFE is just killer. The 500 was light years ahead of the 350, especially in my huge room. It also is a great sub.

M80s VP180 4xM22ow 4xM3ic EP600 2xEP350
AnthemAVM60 Outlaw7700 EmoA500 Epson5040UB FluanceRT85