Hello,
I'm new here and would like very much to add my experience on this subject.
For 15 years, I had a variety of powerful, well-maintained high-end vintage SS receivers (marantz, denon, akai) hooked up to my PMC DB1 speakers and my SVS SB1000 subwoofer. Music was played out of the 3.5 mm line out headphone jack on a mac mini. I thought the sound was really good, but never great.
For reasons unimportant to the discussion, I decided to sell my vintage SS receivers and purchased an NAD C268 power amplifier. Again, I played music out of the headphone jack of the mac mini. No preamp. I WAS ASTOUNDED BY THE QUALITY OF THE SOUND. Literally speechless. I had NEVER heard anything like it, and I have been around a long time. I heard so much in songs i had played for years and never heard. Not only was I impressed by what I had never heard before, but I was completely impressed at all the nuance that was in many recordings. I can't impress enough at how astounding the sound is.
I thought to myself immediately, there is no way I can better this sound! But, then I said to myself "maybe i CAN better the sound"! I thought that I might be able to better my sound with a quality preamp and a DAC. So I spent weeks reading, much of it on this site.
I decided to buy the NAD C 165BEE preamp for $1000. I thought it would pair nicely with my new NAD amplifier. In short, the sound degraded considerably and went from astounding to good/muddy, just like my SS vintage receivers. I really wanted to spend the extra dollars for the preamp and looking for any excuse. I packed it up and sent it back THAT DAY. There was simply no comparison whatsover. Many songs told the difference. The bass at the intro of "Baby did a bad bad thing" by Chris Isaak was so ill defined compared to without the preamp as well as the highs and cymbal sounds and crashes. SO, I have no idea of the voltage of the mac mini's 3.5 mm line out or impedance, but it is MAGICAL straight into the power amp without preamp.
Next, I thought maybe an outboard DAC would improve the Apple mac mini DAC. After reading, I went with the Behringer DAC and the Schit Modi 2 and tried both, again, straight into the power amp without preamp, and bypassing the mac mini DAC. Both were hooked up via USB. Immediately, I heard a sound difference with both DACs. The highs were more separated an emphasized with both DACs, but the sound was harsh and artificial and the mids and full natural sound were missing! The mac minis DAC was indeed spectacular in the extremely listenable natural, yet full sound coming out of it. Both DACs were returned that day. I believe that DACs that "enhance" the sound do so at their own peril. I want no coloration and my guess is that that is exactly what the apple mac mini DAC provides.
So, for me anyway, whether it was the luck of my particular setup or what have you, a preamp took me from magical sound to good and remarkably mediocre sound compared to straight to the amplifier and the sound difference degradation is HUGE and easily verifiable. Lastly, a DAC worsened my sound by making it harsh and grating, by emphasizing the highs probably at the expense of the mids (sort of like when I used to turn up the high freqs on my equalizer when I was a teen), losing the natural and gentle sound that came from the mac mini DAC. Many will say that the DACs I chose were not expensive, but the Behringer has been compared favorably with the Benchmark DAC by a very experienced engineer. But, maybe a $2,999 DAC would do the trick, but I highly doubt it and I don't want coloration to my music anyway, I want faithful reproduction.