What kind of stereo do you have--in particular, do you have the ability to use either toslink or RCA coax digital inputs (i.e., a digital receiver or a standalone digital-to-analog converter--a DAC)?
Think about things in this manner... Digital music exists on your iMac and iPod in the form of .mp3/.wav/AIFF/AAC files. In some cases, the files may be losslessly compressed or uncompressed (e.g., AIFF, .wav) and in other cases, the files may have been subject to lossy compression (e.g., mp3s, most AACs). The audio quality of those files played through a good hi-fi is going to depend upon, most importantly, the quality of the file.
The next thing to think about is how that data gets converted into analog sound to be piped into your preamp/amp and speakers. First, the computer has a generate a digital stream from the file, which may be subject to jitter or noise. That digital stream then has to be converted to analog.
In the case of the iPod, there isn't any way of "intervening" to seize the signal before the iPod converts it to analog. Effectively, you are stuck with the analog outputs. You can get a 1/4" stereo mic plug to dual RCA dongle to connect that output to a stereo RCA analog input. There are some better dongles made--Kimber makes one, for example. But, ultimately, you are stuck with sound that relies on the iPod's internal DAC. Its good for a little tiny box, but its not great.
In the case of the iMac, you can intervene to seize the signal prior to digital conversion. You can use the digital audio out from the iMac. You can also route the digital audio to the USB or firewire ports. Using a firewire/USB device, you then have to convert the USB/firewire digital to analog in another device. For example, you can use something like the Apogee DAC, which has a USB input and outputs analog RCA stereo. Or, you can use a box with a USB or firewire input and a PCM toslink or RCA output. I use this solution--a Waveterminal U24. But the PCM signal is still digital, its just a digital signal most DACs and digital receivers understand. I route my PCM via coax into a standalone DAC. If you don't have a DAC, you could also route it into a digital input on a receiver, if you have one. (If you don't, you are really stuck with using the analog out scenario I talked about with the iPod.)
The benefit here is that you use a different DAC to do the digital conversion. My DAC, for example, sounds immensely better than the DAC in the miniMac. But, that may not always be true. You can, for example, get a high end soundcard. This may improve the DAC processing capabilities of your computer. My personal belief is that inside a computer is a lousy, noisy place to do DAC duties. A standalone DAC may also improve the sound of your CDs, if your CD player has a digital output.
There is also a side issue with how you transmit signals. For example, I've had lousy luck with long toslink runs. To my ear, its better to have a long coax run. Or, even better, a long USB run.
Hope that helps...
