Probably no hard fast answer to your question, but I'll offer my opinions. I suggest you read some of the "setting up a computer for audio" articles by the various manufacturers and/or audio mags. Ayre has some good info on their web site. You don't want the computer doing a lot of other stuff while it is playing music. Spring for one of the software packages that bypass i-tunes processing. I've tried Decibel, Pure Music and Amarra. They range widely in cost and feature set, but all sound a lot better than just i-tunes to me and all have "free trial". I'm not familiar with the Rega DAC, but unless it is optimized for USB use (like the Ayre QB-9), it is quite possible the other digital inputs will sound better. I'm guessing there are some reviews for your DAC out there that discuss this. With the Mac you also have an option to try Firewire - a better interface for music in most cases - if your DAC is compatible. I am currently running Pure Music and Firewire to a Metric Halo LIO-8, and it is by far the best digital ever in my house - although I never had a megabuck CD player for comparison. It certainly blows any of the mid-price stuff I've had from Cambridge or Cairn out of the water. Good luck, be patient, and experiment. Computer audio is almost as fussy as vinyl, but the results can be quite gratifying and the experiments generally cost a lot less than the vinyl equivalent.
mbhintz
mbhintz