I'm still not entirely sure this project is worth productizing or not. But if it is, there is probably room at the high end for something particularly good.

In looking for interface chips, it appears that the DIR9001 is the best overall choice.

For DACs... well, it's all over the place. The PCM1730E is an excellent choice... at $20 a piece.

The PCM1793 is much more reasonably priced, at about $6.25 each. Couple that with an OPA2134UA at about the same price to be the output LPF and buffer... and we might have something worth actually trying to sell.