I'm not the most knowledgeable person when it comes to equipment but I'm sure preamps are just there to contort the sound in some way to help get a specific tone. Like if you like bass and you wanted your music to be bassier you'd add in a bassy sounding equipment in between your DAC and amp so you can get a bassy sound. Or in my case, if I wanted a tubey sound and my second amp was a solid state, I'd go from my computer to my D1 and then use the Tube-out RCA connectors to plug into my main amp. So the setup would look something like this
PC -> USB -> D1(tube out) -> main amp
The "preamp" in itunes is just some software thingy they put in to change up your sound a bit. You shouldn't need a preamp if your main amp does everything you want it to, just get a DAC and run that to your main amplifier.