What you need is a USB to SPDif convertor like the Musical Fidelity V-Link. It would then allow you to output your digital hires files from iTunes via the UYSB out to your 75 ohm RCA coax or BNC in on your Theta DAC. It should (can) sound very close to a good quality CDP but with all the convieniance of iTunes.
Cost to do the above - around 400 US.
Cost to do the above - around 400 US.