The HD800 has always been considered a hard headphone to drive correctly or to its potential. The claims are always made that it is thin, not so much complaints about the soundstage but all open up as you get an appropriate amp for it. I run a Eddie Current Zana Deux and they preform amazingly. It is geared toward high impedance headphones and is tube. Kind of expensive though. I also have a Violectric V200 which is cheaper and has a warm character. Both drive the 800s perfectly but you can tell the difference in performance fairly dramatically even between these.
Portable amps can drive the 800 just not so well thus your issue. I have an older Lisa III amp that drove them very well but it is almost to large to be portable, and still it is not as good as the others. I have a Pico Power amp that is portable that could theoretically drive the 800 but why bother, i have portables that it drives better.
All I can speak is from experience, go to a meet or send them back, just be advised that they are probably doing what they should do just need more "umph".
Oh, for dacs i use a Chord Hugo most often now. I also have an older dac that i really like the sound of that had a couple of thousands of dollars of modifications done. It is a little outdated but i love the sound signature. Dac's are hard to evaluate in my opinion because hard to put them on your system and know how they will work with your gear. I do love the Chord products i have heard though.
If you have any other questions feel free to pm I think this is all i want to talk about the 800 in a Pandora thread though-off topic for Pandora Hope users. Don't know how much more i can add anyway, good luck in your search.