If you have the new iMacs, I'm assuming you're talking about the anodized aluminum iMacs with Intel Duo cores. If that's the case, then we both have the same model
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The computer comes with the newer Intel High Definition Audio integrated sound cards. The specs are alright, but I'm noticing slight differences from the Audigy 2 zs PCMCIA card I used to run on my Dell Laptop through the same DAC setup I have. More base. At this point, I'm more concerned about the card outputting bit perfect 44.1 than anything else and I haven't seen anything conclusive on this yet. Apparently the Audigy 2 zs upconverted to 48Khz on the fly without properly filtering which cause artefact issues in the high end of the spectrum.
The set up I used to run on my DELL also works on the iMac 24".
I recently installed Cog and love it. iTunes isn't super for playing different types of music (it's good to organize, but that's about it) and if you don't want to be stuck in a proprietary format, then apple lossless isn't the way to go.
I roll all my music on FLAC through Cog and output to a Zhaolu D2.5A through the SP/DIF (optical out) jack.
The Zhaolu is a DAC/Amp combo and it's actually difficult to beat for the price. I tweaked mine a bit and did some standard mods to it (caps bypass on the analog stage and switched the op amps to LM4562NA) for a dryer, more detailed, analytical sound.
It's a nice little DAC and is a very interesting candidate for the price tag you're looking at. I would opt for the discrete amp upgrade to get a better sound experience with your headphones. That would probably bring you right around $300 (a bit under).
Also, the there is nice community of DIYers that like to tweak this DAC like mad to get all the juice they can out of it. In the future, if you want to use it as a DAC only, you can always up the analog stage to output discrete with something like the Zapfilter, removing the discrete amp to avoid interference with in the analog stage and out to an external headphone amp.
iFi Audio
Also, the MUSILAND seems interresting and from what I understand, it's a USB DAC, so you end up bypassing the Intel HD Audio altogether, which may be a good thing if you're not sure about it's quality or if you don't like the way it sounds. I never actually tried the MUSILAND, but it does look interesting for the price and I heard from other Zhaolu fans that it sounds very nice. Unfortunately (or fortunately), the MUSILAND is a DAC only and does not contain an integrated headphone amp.
iFi Audio
So my setup:
iMac -> Cog -> Zhaolu D2.5A -> Grado RS-1
Regards
El Condor