Alright, so a noobish question, but I want to make sure I am doing this correct. I have my desktop amp (Head Box II) hooked up to my iMac's line out and my amp drives the SR80i's. So, iMac line out > 3.5 to RCA to amp >headphones. Is that correct or am I doing something wrong? I've heard that you need to hook up the RCA to a USB connect to bypass the soundcard in the computer? I don't know! Any help is appreciated!
What you're doing should work fine. If you're not hearing static/hash, then keep using it.
You can use an external DAC if you want. Your Mac has a mini-Toslink jack in the headphone jack, so you could use an optical cable to connect that to a DAC. Some DACs have a USB connector, too.
Then you'd connect the DAC via RCA cables to your amp.
I'm not a big believer in spending a lot on digital sources - they all sound about the same when you level-match them. So if you're not getting any noise (obvious stuff, not "golden ear" things) then just jack the computer into the amp and enjoy.
You connect a USB DAC to the computer then use it's RCA (or 3.5mm phono, if that's what it has) to feed the amp, this bypasses the onboard sound card. What we call a stand-alone DAC unit (since every digital source with an analog output has a DAC chip) for traditional HiFi systems is basically a soundcard with a separate chassis and PSU (even if it uses a USB charge-pump circuit.)