Just read through this thread - learned a ton about my device! Glad you're all enjoying it as much as I am. I picked up my HDP in March on head-fi's classifieds for $315 barely out of the box.
As a headphone amp, much has already been said, and it all comes right back to the OP - "It's awesome." I've compared it to my Fiio E7 which I use for the office/portable ventures, as well as to my Qinpu A3, which is a tube/hybrid integrated/headphone amp. I also used my friend's Zero DAC a number of times. The HDP is the clear victor for driving my ATH-M50s, my only audiophile cans (pre-redesign/price hike). I didn't have any speakers until June, so I spent a solid two months loving this amp/dac for nearly all my music listening.
I'm posting mainly because only a few people have talked about using it as a preamp/dac for tube systems. As of June, this is my speaker setup:
Macbook -> HDP via Optical -> Qinpu A3 -> Klipsch Quartet Floorstanding Speakers circa 1989 (part of the renowned Klipsch Hertiage series)
I picked up the Qinpu for $140 with light use and the Quartets for $400 in mint used condition from the original owner - an $855 setup if you ignore the cost of the computer. So anyway, my thoughts as a speaker listener:
In this thread, some have commented on the HDP's sharpness and brightness. It is slightly there when I listen through the M50s, but I actually prefer a bit of brightness (nothing like actually feeling a cymbal crash), and I never found it overwhelming or harsh. It can be fatiguing, though, as I don't like to keep the cans on for more than a couple hours at a time. I'm always like that with headphones, though, so maybe it's just me.
Regardless, I have to say that the warmth of the tubes make for an awesome best-of-both-worlds scenario for me. I get the detail and sharp clarity of the HDP at most frequencies, and I pair it with the tubes to 'warm up' and shape the sound (roll the tubes to customize it a bit!). Then throw that signal into a classic hi-fi speaker like the Quartets and you're in heaven. Perhaps I'm not critical enough, but I don't feel I'm lacking anything in this setup, besides maybe some power in the highest of registers, but that could just be the tubes. Perhaps when I'm older I'll be listening to more classical, refined music and I'll want high powered, super detailed equipment, but for now, I'm loving the unique shape that my sound has taken with minimal power and fairly low investment.
For me, the HDP is a critical component as both a headphone amp and a speaker preamp - true bang for my buck. I don't anticipate needing to replace it for quite some time.
