The DAC on the HDP is a little better than the amp section, and it beats my other $400 or cheaper DACs on hand with a line-out (HR Micro DAC, Pico DAC, iBasso D10, iBasso D4, Nuforce uDAC-2). Only the DACport comes close, and I found in my comparisons that the HDP DAC has a slightly better sense of air and micro-detail and extension, although the DACport sounded a little more refined. The DACport has no line-out and I was using it as a pre-amp via the headphone jack output, into my Amphora, WA6 and eXStatA when I did the comparisons. The $500 Apogee Duet might be slightly better as a DAC, as I once said the Duet was 98% of my Apogee mini-DAC which is also better. I don't recall a lot about the Duet headphone out except that it was slightly better than my mini-DAC headphone out (like the HDP) but the Duet headphone out had too much hiss when I tested it.
I think that the HDP is worth it even if it was only a DAC, and the headphone amp is on par with or better than some other $200-400 headphone amps, reminding me the most of my balanced RSA Protector amp. In side by side comparisons the RSA Protector in balanced mode with HD600/800 and HE-5 LE sounds almost the same as the HDP with these same phones using a single ended adapter. Power-wise the HDP can drive the HE-6 and the Protector can't. Without a balanced cable the Protector is inadequate for the LCD-2 as well, while the HDP is a good match for those.
[EDIT - I want to add that the lesser DACs I mentioned in my post all sound very good via line-out. The HDP is not jarringly different at a similar price point, but I felt a better sense of air and space and micro-detail with my 24/96 recordings from the HDP than the similarly priced or cheaper DACs. And, with my SR-71b amp I was definitely hearing better bass extension with the HDP as source than the Pico DAC that I was using previously (that difference in bass depth went away when I switched to the ALO Amphora amp, so it's a synergy issue with the SR-71b). Basically the sound plus features and flexibility of the HDP DAC put it over the top for me vs most of these other DACs].