The REL Storm III is a great sub. It will be better than your 603s in the bass department. That said, you could keep the REL forever when/if you ever upgrade your mains. There are folks using the Storm successfully even with the B&W Nautilus 804 and 805. If you do not think the Nautilus or something similar is in your future, the Stratus III is also fine for your 603s and you can save $500.
I am talking about music. If you want one for movies, then the REL Q series (Q100E, Q150E, etc) is even better for that, or the Velodyne.