If you can afford it, get a DAC. The amp could be used out of a line out on your computer (or something else with a line out) but it won't be anywhere near it's full potential and may even only help minimally without a DAC. From what I've read I bet the HRT would be a fine choice, or you could go for an ODAC for a matching set
And headphone out is not the same as line out. The former is already amplified, so you'd be amplifying an amplified signal, so go with line out (unless headphone out is the absolute only thing you have, then I think 50% on that is the recommended volume). This only applies without a DAC though. The DAC would connect by USB, optical or coax, getting a digital signal that has no amplification, but would be converted into a line out that goes from it (the DAC) to your amp.