The D-777 is very well thought of. I'm sure it'll drive the Etys fine from the headphone jack.
All headphone jacks have a built in amp to deliver power into a headphone - but you might get better sound quality if you use the line-out option (turning off the internal amp) and instead use a dedicated portable amplifier. You get higher quality electronics and usually a better designed headphone output by using a dedicated amp.
What kind of budget do you have for a portable amp? Some of the popular ones recommended here start cheap, from DIY CMOY or MINT amps, the PA2V2 ($60 from www.electric-avenues.com) all the way up to the Ray Samuels Hornet and SR-71 (www.raysamuelsaudio.com).
Just FYI - if you're going amped, it's preferable to use the ER-4S model, which has a better accuracy rating. You can also convert the ER-4P to the S model with the converter cable (basically a resistor) sold by Etymotic and Headroom.