See here and here. They seem to have a sensitivity of 98 dB SPL @ 1mw, 56 ohm.
Sometimes however with more power, you will hear a greater ease of presentation or perhaps more detail because an amp provides more juice for a headphone to properly do its thing. Just keep an open mind and realize that it can go either way. Win with some things. Lose with others. Trade offs.
These guys sell really cheap DACs in various configurations. They seem like a good value, haven't tried them, though.
It's me again. Man, this problem will never stop. I feel guilty for posting this often here.