Great point Plhart.
Adding a subwoofer would be an excellent choice.
Nick250,
Remeber speakers do not "clip". Amplifiers do. Speakers can distort distort heavily due to amplifier clipping.
Chromis,
Distorted signals tend to sound louder than clean ones. The differences can be fairly significant too. Your 603's are great high fidelity low distortion units. This could be part the reason they do not sound as loud as your friends' systems. Also, your friends' systems may integrate into their rooms better (than yours does in your room), taking advantage of bass modes, thus increasing the "wow" factor. Essentially, in listening to their systems in their homes, you are evaluating how that syustem interacts with that actual room. Not that system itself. One of your friends' systems may or may not perform nearly as well in your room.
I would suggest a good 12" subwoofer as an addition to your system. Your receiver has dedicated subwoofer outputs, but I do not believe that it has a large/small switch or crossover. I may be wrong, consult your manual. I would suggest getting a sub with speaker level inputs.
Good Day