First, I don't see much benefit in having those big 603's as rear surrounds...they'll do nothing for 2-channel music listening, obviously, but even with movies or multi-channel music I don't see them making much difference given the way music and movies are mixed and the content that is distributed to the back channels with multi-channel music or movies. It's usually just ambient sounds, environmental sounds, which will gain very little in going from the 601 to the 603. And do you have a large enough room that it won't look awkward having large tower surrounds?
So the question becomes, how much difference between the 603 and the 604S2 - the main functional difference being the extra woofer. You already have a decent subwoofer, so it's not as if you're desperate for low end help (or you shouldn't be). Is the issue that when you listen to 2-channel music, you just don't like the balance presented from the 603's and you want more low-end presence? Unless this upgrade costs you very little, and the 604S2 are being dumped in your lap for a song, I don't see this changing your system all that much. Again, unless you don't like the way the 603 present 2 channel music, but you indicated you're not unhappy now.
What I don't know is whether the tweeter advancements between the series 1 600's and series 2 were such that you'd do this just for the tweeter change, if there even were many enhancements between series.
This may be an upgrade just for the sake of upgrading and curiosity - I don't know that it'll change your music all that much. Maybe someone who's experienced both series of the 600's can chime in...
So the question becomes, how much difference between the 603 and the 604S2 - the main functional difference being the extra woofer. You already have a decent subwoofer, so it's not as if you're desperate for low end help (or you shouldn't be). Is the issue that when you listen to 2-channel music, you just don't like the balance presented from the 603's and you want more low-end presence? Unless this upgrade costs you very little, and the 604S2 are being dumped in your lap for a song, I don't see this changing your system all that much. Again, unless you don't like the way the 603 present 2 channel music, but you indicated you're not unhappy now.
What I don't know is whether the tweeter advancements between the series 1 600's and series 2 were such that you'd do this just for the tweeter change, if there even were many enhancements between series.
This may be an upgrade just for the sake of upgrading and curiosity - I don't know that it'll change your music all that much. Maybe someone who's experienced both series of the 600's can chime in...