Has any one used the optical out with any DAC?
Not extensively. I've tried it once or twice with the Hugo, but found it redundant. The reason I like the QP so much is that I don't have to use the Hugo to get good sound except maybe with the LCD-Xs. The X5 needed the Hugo, but in my opinion, the QP doesn't.
I might try again later today with a classical piece I have in mind, and will report back if you like.