If you want to get good audio quality out of your Mac, you might want to forget about using it to drive a pair of small add-on "computer speakers" and instead, run the output of the Mac into a good audio system.
This can be done by buying an adapter that has a male mini stereo headphone connector at one end and a pair of female RCA connectors at the other end. Plug a pair of good one-meter audio interconnect cables into the adapter and then plug the other end of it into the Mac's headphone jack. Plug the other end of the interconnects into the input of your stereo preamp (or integrated amp or receiver).
When a system is configured like this, it's best to only use the volume control on the audio system, and keep the volume on the computer wide open. Since the Mac has an electronic volume control, when it is set wide open, it is essentially out of the signal path.
My main speakers and subwoofers are on either side of my computer desk. Here's the system configuration:
Mac Mini => Cary Audio Design CAD 5500 CD Processor (a vacuum-tube preamp with two pairs of outputs)
Cary CAD 5500 preamp => Parasound 75 Watt-per-channel power amp => two PSB Century 800i speakers
Cary CAD 5500 preamp => NHT X1 active crossover set at 80 Hz => Mackie M800 175 Watt-per-channel power amp => two JBL 2231A 15-inch woofers
When setting up any music playback system, as a general rule it's best to keep the signal path as simple as possible. The audio for my TV is fed directly from the cable box's audio outputs to another pair of inputs on the preamp.
