Hi G, No volume control on the new Esoterics. The D1 does not have it and the upgraded K-03X & K-01X no longer are equipped. When I had my K-01 I used it direct for awhile. At the time I had a Bryston preamp for multi channel, which is a pretty good preamp, and the K-01 was amazingly good sounding, but, After awhile I ran it through the preamp. I mostly listen to CDs and SACDs, so I would think that with Hi-Rez straight through the DAC could be the best hookup.
When I upgraded to the D-02 I had the Esoteric C-02 Preamp, and without doubt liked it better with the preamp.
And now I have no choice! there is no volume control offered. I am running the Ypsilon preamp and love the sound mated with the Esoteric. It clearly helped the D-02 balance out and I liked the darker overall presentation.
With the D1/P1 I am very satisfied with no filters using the ES-Link 4 HDMI protocol. The music just flows through these machines. Overall much more resolution than the D-02. I always felt that the D-02 was as good as it gets with retrieving detail. Finding the information on any format and playback was really good, but better with some stuff than others. When I got the Ypsilon preamp in line, that was as good as I had in my system, but Redbook didn't have the texture and resolution that the D1s provide. The D1s are great all around as any DAC I have heard, though I have not heard many of the top rated ones, I just don't feel like I am for loss, and can concentrate on other parts of my system to match the strengths of these babies. They may not be for everyone, but I would bet that more listeners would be happy with these Esoterics, and that may not of been the case in the past. For me they excel at everything Esoteric is known for, and they do well what they were not known for with the D1s. Best of dynamics, great detail, top of the line resolution, ultra super quiet noise floor... music arrives from nowhere, effortess flow, extremely smooth and incredibly lifelike presentation. I have not experienced any of that glare or occasional brightness, that plagued my K-01, just delightful, satisfying music!