I just started playing piano and gained interest in digital audio. My Mac Pro having been used mostly for digital photography so far, it only had fairly basic speakers attached (LaCie USB speakers designed by Neil Poulton).
I was thinking about investiing in a small desktop amp (NuForce Icon 2, project, ... SMSL) together with a pair of good monitor or dipole surround speakers.
Many such amps offer a USB connection, but I noticed that my current USB speakers often suffer from spurious emissions.
Is this a Mac USB audio issue? Would a DAC connected to the digital output of the Mac Pro sound better?
The big question you are asking is where to put the DAC. There is one inside the computer that drives you line-out/headphone jack. Then you can buy an audio interface with a DAC inside and as you say conect it with either optical or USB.
My opinion. If you are musician with a digital piano you might like to use it as a MIDI controller or record your playing. So buy an audio interface that is designed for use in a small recording studio.
I like this one it has a built-in MIDI interface and will do 96K x 24-bit audio in both directions and has 48 volt phantom power for mics if you need that. and the price is not bad. It has balanced outputs so you can drive studio monitors that typically have balanced inputs.
http://www.sweetwater.com/store/detail/AudioBoxUSB
The top video is really good, watch it to the end and read the titles.
SOme people like the Apogee Duet. It has some big faults and one advantage. The faults are tthat it uses a dogle (octopus) for cable connections, it has only ONE knob that is multi-fictions, the sound quality on playback is no better then units 1/2 it's price. The one good feature is low noise recording. the INPUT section is good. However the input section on the "audio box" is good too. ANd then you you own any $900 microphones? If not you want notice as the mics are the weak link.
All that said, none of this matters so much. What REALLY matters in other of importance, most importance first is
1) room acoustics and any acoustical treatments you have done
2) placement of the speakers relative to both the walls and you.
3) the brand and model of speaker
4) the electronics (audio interfaces, amplifiers, cables and what not) you use to drive the speakers
Think the entire system through, especially if you will be doing any music production/recording using your piano.