Hi Jon,
I also believe that the reconstruction filter is necessary. The top scope trace is after the eval board's filter (at its output mini jack), the second one below is directly from the chip -- both are 1 KHz sine waves, 30 dB down from max. As you said, there's enough dynamic range to provide a clean output. even at this low level -- but only when the out-of-band noise from the sigma-delta DAC is properly filtered.
Dan: The required DAC output filter (reconstruction filter) is described on page 21 of the ADAU1701 data sheet. The passive version is similar to what's in the mini eval board. In the active filter version shown, C8 should be 470 pF, not uF.
Best regards,
Bob