I'm referencing only Macs on this post. PC's are probably the same but I don't know that for a hard fact.
So anyway, music, stereo in particular. Higher end stereo. That means it's not an iPod or iPhone.
Stereo gear runs the gamut of quality and can be insanely expensive and that once you get to even the mid range of equipment you really have to "tune" the room where you listen to music to take advantage of quality gear. I'm not getting into that stuff though.
Here's the basic rule for stereo sound quality. A good mid range to high end stereo system will not make poor recordings sound better. It's exactly the opposite.
A high end stereo is typically intended to be "transparent" in that it imparts little or no influence on the recording so you hear the music exactly as the artist and producer wants you to hear it. The downside of that is that the equipment does not mask poor recordings and if there are flaws such as boomy bass or harsh tinny cymbals or muddy vocals - that all comes through. It is annoying when that great tune that you love in your car becomes nearly un-listenable on your "good" stereo.
Low end gear such as ear buds and car stereos mask flaws and there is usually so much ambient noise when you listen to music in that way that clarity is almost the least important aspect.
So, on to the computer.
On the back of your Mac is an audio "out" port. It's a standard analog 1/8" stereo plug and you probably have small computer speakers or a headphone connected to it. Here's the cool thing - that port is also a digital fiber optic output port. All you need is an adapter that allows you to connect a standard Toslink cable to your Mac.
A quick Bing search brought up this, http://www.trianglecables.com/toslink-digital-optical-to-3-5mm-.html
Plug that adapter in to the same 1/8" output port and then plug a Toslink cable into that adapter and you're now outputting a pure digital signal! It's so simple a caveman can do it.
Here's an important note that no one bothers to explain. Almost all music now is digital and needs to be decoded, if you're running a purely analog signal from a LP to an amp then you're either using really old equipment from the 80's or earlier OR you're running stupidly expensive modern equipment and you're just wasting time reading this blog.
When you run an analog signal (via the standard 1/8" plug) to your preamp, processor, amp or receiver then the signal decoding happens at the playback source, which in this case is the Mac.
If you output a digital signal through a Toslink connection then the Mac is sending a pure, unaltered digital signal to the next device in the chain. What that means is that the preamp, processor, amp or receiver is doing the decoding. This is really important because not all decoders have the same capabilities. This is true for any DVD/CD/BluRay/or whatever player too.
I believe we all get sold on the idea that a digital Toslink cable provides the optimum sound quality. That is not at all entirely true because it is completely dependent on how well that signal gets decoded and interpreted - and it's totally subjective as well.
The only way to know is to test both analog and digital pathways and listen to it to determine if one sounds better than the other. This is no small matter, the decoding is a HUGE part of how the music sounds in the end.
Generally speaking a device or piece of software specifically designed to decode digital signals will sound better than a decoder built in to a receiver or even your CD, DVD or BluRay player. A hardware unit is going to be called a processor but many preamps also decode. There is software available for you Mac that can also act as a processor, but you have to be careful about how well it gets along with iTunes.
OK, on to the next topic. iTunes.
ITunes is great. But it's great for a reason you may not realize. iTunes is the ONLY music playback program that I know of that allows you to create and assign a custom EQ setting for each and every song and have that EQ automatically activate when the song is played.
There are tons of playback programs available and most of them have an EQ option, but all of those EQ settings are "global" settings and therefore that same EQ is applied to every song.
If you've listened to music at all you've probably noticed that there are ZERO standards for how a song or album or disc is mixed and therefore some songs sound amazing and some are just pure crap. Maybe the tune has muted vocals, bass that is too thin or too heavy, high frequencies that are too bright and harsh, or not bright enough.
In those cases you really HAVE to apply a custom EQ to make some recordings listenable. Because of the lack of standards, a global EQ is just about as worthless as you can imagine.
I'm hugely oversimplifiying EQ. It is very common to have a global EQ that affects the stereo system's interaction with the physical walls and ceiling and dimensions of the room and that is entirely different than custom EQ that is applied to a specific song. This is altogether a different topic that will not likely ever be discussed on this blog. Nonetheless, I'll stick by the statement that a global EQ that is applied to each and every individual song is worthless.
Just as a brief side note, digital music comes in a number of forms and the most important aspect to consider is bit rate and bit depth. The higher the number the better. For exampe a standard file you'd buy from Apple contains between 128 and 256 kilobits of data per second (kbps) of music and be around 5MB in size. A standard CD file is going to contain 192 to 320 kbps while the newer "high resolution music files are going to run between 2935 up towards 64,000 kbps and be around 130MB per file. There's a huge difference here.
If you run basic AAC files that you buy online a digital connection is not worth your time to do. If you run the medium resolution files from a CD then you're likely to hear some difference from a higher end connection. If you run the high res files then you must have gear to take advantage of it. iTunes allows you to look at the file sizes and bitrates. It's worth a look, you may be surprised at how low resolution the music files are.
The next topic is another gadget in the Mac world which is a small program in the Utilities folder called "Audio MIDI Setup". This allows you to specifiy the bitrate that the computer will output audio files. I think by default it outputs the standard CD bitrate of 44/14 but you can change that to the high resolution output of 96/24 and you really should do that if you're playing anything above a standard low res AAC file. If you do not make that change in the Audio MIDI Setup then high res music files get dropped to a lower bitrate.
Getting back to the EQ feature of iTunes. Why is that important? I referred to the lack of standards in how music is mixed or produced which has a huge effect on how the music sounds to you. A custom EQ can do a lot - a lot - to improve the sound quality of poor recordings.
I listen to older music quite a bit, from the 70's through the 90's. A few recordings that were originally done on LPs and later on CDs are pretty darn good. However most are pretty poor. I "rip" tunes from CDs a lot and sometimes rip from LPs as well. I think every single song needs some level of EQ to make it sound good to my ears. It may be just a slight bump on the low end or it could be a treatment that affects much of the song's range.
This is a lot like digital photography. The file that comes out of your camera can look pretty good, but it's really, really, really, rare that a photograph is perfect as is. Any photo is going to benefit from cropping, or a change in brightness or contrast, or the colors need to be tweaked, or a little bit of sharpening and so on.
Music is exactly the same way.