Enjoyed reading the review, but I didn’t find any mention of the output impedance of the HDV-820. If it is like that of the predecessor, the HDVD-800, it is 43 ohms, which is great for the 300-600 ohm (depending on frequency) of the HD800, but is awfully high for most low impedance headphone that are around 32 ohms, such as all the Grados, the Fostex TH900, the Ultrasone, the HiFiMAN HE1000 and HE 500, and many others. Rule of thumb is that you want the output impedance of the amp to be less than one-eighth the input impedance of the headphones, or 4 ohms amp output impedance for these 32 ohm input impedance headphones.
This fact has kept me from using my HDVD800, which I find utterly fantastic with my HD800 headphones (found no amp better, and despite concerns by some of its “so-so” DAC, found no shortfall in comparing to Hugo and MHDT Stockholm dedicated DACs), with headphones other than my HD800. I have a wonderful but single purpose Sennheiser chain... HD800, HDVD800.