Open up the audio settings in your iMac utilities folder and set the output to whatever bit depth and sample rate that your files are, probably 44.1KHz and 16bit. This will ensure that the iMac doesn't resample the music files before being sent to the DAC and is termed 'bit perfect'. Also, I would turn off all iTunesprocessing, such as EQ.
iTunes will not switch sample rates on the fly and will need to be manually reset if you listen to any high res files, such as 24bit/96KHz.
Personally, I wouldn't get hung up on it all, just listen to the music.
As you have now listened to the DACs in both the Marantz and the DAC magic, you will be well placed to be able to determine if there are any differences, by switching between the two. This will give you some idea as to whether or not you need to spend more money on a DAC.
Edit: You could also try to detect differences between audio sample rate settings to see if the iTunes resampling is audible. If it isn't, then that's one less thing to worry about.