If your imac has optical, I'd opt for that. It will pass digital to your amp, which will then function as a DAC (digital / analog converter). However, I have no clue what ports are available on the imac you have.

If you don't have optical on the imac, do you have SPDIF? You can pass that to the amp as well, and have it decode for you. SPDIF is just the coax/copper flavor of optical. They're both digital feeds. Send one of these to your amp and have it do the decoding for you :) One of the advantages with optical is that electrical signals can't interfere with it.

If you're using the regular 1/8 trs headphone jack on your soundcard, you're missing out on some definition and quality. Using a real hardware DAC, either the one in your amp, or a dedicated outboard USB DAC, you'll hear a big difference. Internal sound cards don't hold a candle to a good, quality dedicated DAC.