Hi XavierL13,
I totally hear you about the clutter, but I totally understand that sound is an issue as well. I am a professional audio engineer, but I don't need a full blown system in my office. It's just to distracting during work anyways.
I actually recently found a really cool Bass add on made for the iMac. Chech it out.
http://twelvesouth.com/products/bassjump/ It's USB powered so you even eliminate a power supply. And while you are at it, the same company that makes the speaker also makes a little mounting bracket that goes right on back of the iMac, so you can even hide the sub behind your screen.
http://www.twelvesouth.com/products/backpack/
Again, I garantee this will not create massive sound, but it will ad a lot to your computer totally mac style!
How is that for a solution? Just as a disclaimer, I don't have this system, and I don't have an iMac, but I wish I had it 🙂.
Cheers, Hans