I wouldn't spend money on cables unless I could repeatedly hear a huge difference when I didn't know what cable was in the system and with matched volumes. Cables may or may not improve sound. But even if they do, the difference is very subtle and not worth their huge expense.
If you are determined to spend more money on your system, I suggest upgrading the source and/or amp in that order. Note I'm not saying what you have are bad, it's just that's where you get the biggest upgrade bang for the buck. The best bang for the buck is to spend the money to get well-recorded music that you like, but then you won't get the warm fuzzy feeling you'd get from buying more equipment.