Mac iTunes to hi-end audio.

At the risk of being a newby and stupid... What is a good option for getting all the music I have on iTunes to my "hi-end" listening equipment. I haven't upgraded my "good stuff" in 20 years or so. Amp is Boulder 500, Preamp is Boulder and I have a Theta DS Pro basic ( single RCA digital input ). I have a Theta Jade transport for CD and some vinyl. For iTunes, currently I convert to lossless and use the Mac audio out to Aux RCA (cheapy cable) on the preamp. Sound is acceptable but can I step up to CD quality. Not looking for a discussion of what qualifies "quality" thank you. Not looking for a miracle either....perhaps just a more modern DAC with a USB in to Analog out to replace the Theta DAC.

I use the HRT music streamer II. It is a great low-cost option, and I don't feel like I'm missing out by having a cheaper DAC in the system.

I also use Channel D Pure Music as an add-on to Itunes. It rides over the top of Itunes. It does sound better than just Itunes.

You would be very happy with these options, in my opinion.

Good luck!
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.