I don't really see how someone could say the SP2500 is lacking in bass. Actually any decent 2.1 setup for that matter; I mean, you have a dedicated subwoofer. I have the Klipsch Promedia 2.1, the newer one that I believe has a weaker sub(?), and I have the bass dial turned down to less than 1/4 of it's full travel. When I turn the dial all the way up, there is so much bass that I can feel the back of my eyes vibrating and it gets me feeling a bit nauseous. Of course the subwoofer isn't as powerful as the one with my home theatre speakers; the Klipsch sub won't be shaking any windows or toppling pictures on the shelves. But at that point, you wouldn't even really be able to hear your music, not to mention your neighbours would want to kill you.
It's good that you are satisfied with what you have, because at the end of the day thats all that matters. But I am most likely going to have this audio system for many years to come and for what I was spending, I wanted to have the best I could buy.
On your Mac you will be fine using a standard 3.5mm jack, there is no need for a special cable.
Thanks for clearing that up for me.