I think you actually have a system that can easily be improved on within your budget, especially with respect to the inadequate subwoofer you now have..
The driver of the sub is way too small, it does not have enough power, and even the manufacturer only claims response down to 40 hz, which does not really make it a SUBwoofer. You need a lot more power and a 12-inch driver to get good response well below 30 Hz.
I suggest that you go to Amazon, spend $389 on the Klipsch RW12D subwoofer, and then see what your system sounds like. You need the new subwoofer in any case. You can decide if further changes need to be made after you get it. First thing first.
Set your subwoofer to roll off at around 60 Hz, and use your front speakers down to 60 hz.
I think the improvement in your system will amaze you.
If you try this and still think you want better floorstanding speakers, The RTi A5 would be the next logical step to get a major improvement. The main driver of the RT600 has been commented on by reviewers as a weakness with regard to adequate power and clarity.
If the RTi A5 is over budget, the TSi 400 would also be a significant improvement over the RT600, and is less money.
Edited by commsysman - 7/19/12 at 4:05pm