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.This was true years ago, however today all channels are discrete, uncompressed, and full bandwidth. A sound may move through the room in anyway imaginable, or be fixed at any place in the room, therefore any two or more speakers may be used to create the sound. Take a look at this picture. Obviously, that jet would sound the same using identical speakers for the front and rear, however using very small rear speakers compared to the front will result in that 747 jet on the front sounding like a much smaller aircraft on the rear. Ideally the best home theater would have identical speakers all the way around, however this is usually not practical without a dedicated room. Take a look a this article, especially the last line. Obviously, action movies will benefit the most from the upgrade, and IMHO, there will be much more than a "slight difference".
So the question becomes, how much difference between the 603 and the 604S2 - the main functional difference being the extra woofer.Actually, more than just a extra woofer. Series 2 improvements over Series 1 include the following,
1) Redesigned woofers for a much stiffer cone.
2) Redesigned midrange with added phase plug.
3) Redesigned tweeter with Nautilus technology. This also applies to the CC6 S2.
Again, IMHO, there will be more than a "slight difference" between your DM603 and the DM604 S2.
So yes, I recommend the upgrade, but only at a reasonable price, and maybe negotiate to take the CC6 S2 for a better deal.