Reporting back. I purchased four DAC's to try out and have been listening for a few weeks. Connection to the DAC's through an optical cable and then RCA outputs through the Schiit SYS pot, then to the NAD amp, and out to XTZ 95.24 speakers + Dayton SUB-1000.
Audioengine D1 $170 (Price has now dropped to $130)
Small and simple with a headphone jack. Variable output on both the RCA output and 3.5 mm headphone jack. The sound is bass heavy. I had to turn down the volume on my sub-woofer with this DAC. It sounds fine, nothing obnoxious, but nothing spectacular either. I was less interested and involved with the music than with most of the other units.
Cambridge Audio DacMagic 100 $200
Simple DAC, pleasant sound, fixed output, no headphone port. Better tonal balance than the Audioengine D1 (less bass, more treble). Inspires more involvement with the music than the D1 and worth the extra $30.
TEAC UD-301 $400
Very nicely built, fixed/variable output option, handles DSD, upsampling if you want it (easily defeatable if you don't). Painfully bright sound. This DAC is unlistenable. All I can hear when listening to this DAC are cymbal strikes. I tried switching between variable and and fixed output options and both are harsh.
NAD D 1050 $500
Nicely built, horizontal or vertical orientation, headphone jack, fixed level on RCA outputs, volume knob controls headphone output only. Sound is balanced, perhaps slightly warm (although I may have a hangover from the TEAC UD-301). Subtle details are present but without any harshness. Great image and depth, very musical and involving. Dire Straits' "Your Latest Trick" made the hairs on my spine stand up.... I find myself wanting to turn up the volume with this DAC (in a good way). Some of the other DAC's I listened to inspired me to turn down the gain in order to decrease the pain.
Guess which one I'm keeping....