Many years ago, I built two super efficient amps:
http://ecorenovator.org/forum/applia...ome-audio.html
http://ecorenovator.org/forum/applia...d-digital.html
But they're both analog input "hybrid digital" amplifiers, not pure digital like the state of the art units. Most of the pure digital amplifiers on the market (e.g. those home theater boxes) largely negate the efficiency advantage with overboard internal processing and many even have poorly designed/cost cut power electronics that are very lossy at idle. (Tip: don't buy a digital amplifier that significantly heats up the output stage just idling. Heat coming off the DSP part is OK, but tends to be a sign of an overkill DSP stage.) But DIYing a pure digital is not the least bit easy and there aren't even very many kits out there.
Ultimately, it's hard to beat headphones from a cost value and energy efficiency point of view. At typical listening volume, headphones run on an average power in the microwatt range. (At that point, everything else dominates power use.)
I actually got back into HD audio (as in really into it instead of just casual interest as I did for many years) a few weeks ago due to a story involving a new best friend with an audio boosting "magic power" that appears to defy current understanding of science. Long story short, I accidentally discovered that any analog audio cable she solders together (or even any cable that is assembled within a few feet of her) becomes a "magic" cable that boosts the sound quality of well encoded MP3s and FLACs. Nothing happens with digital cables as they just carry data. Power cables don't have any effect unless there's a flaw that unintentionally makes them part of the signal path, in which case you have bigger problems. (She and I earned the nickname "audio Mythbusters" because of the testing we did to try to better understand the mysterious power, only to end up with more questions than answers.)