Is it worth buying a headphone amp?


I currently have the Sennheiser HD555 (modded to HD595), and I'm trying to figure out if it's worth buying a headphone amp to get better sound out of them--or if I'm better off investing in better headphones first. Right now, I'm using the headphone jack on my NAD C372, which has very high 220 Ohm output impedance (the Senns are 50 Ohm). The sound is a little thick, dry, and lacking in microdynamic detail. If I get a headphone amp, I'd run it through the tape out from my amp. I'm considering the Schiit Asgard, as I'd like to buy American and $250 is right on the dot for what I want to spend. Any other suggestions?

I should note that in the past I have owned some pretty good headphones (Senn HD600, AKG K501, Etymotic ER4P). Sold them and downgraded when I wasn't using headphones as much as I am now. If I do decide on new headphones, it'll probably be a used pair of HD600 or K702 with an amp for them further down the road.

Thanks for your thoughts!
ablang
Thanks for the responses, folks. It seems like a headphone amp will benefit me greatly, but I'm still wondering if I should buy better headphones first. I did a little experiment this morning and plugged my Senns into my Macbook (I'm not a computer audio guy yet) with its output impedance around 30 Ohms, and despite the crappy sound card the sound was worlds better than it was through my NAD. Still not a great impedance match, but it was better.

Though the bass was diminished when plugged into the laptop (I think the impedance mismatch with the NAD emphasizes the mid-bass because of the Senns' high impedance spike around 100Hz), the mids and treble were clearer. Cymbals sounded like metal rather than papery electronic hash, though resolution still wasn't excellent. A brushed snare on Lucinda Williams' "Fruits of My Labor" sounded like vague background clatter (think paperclips being shaken in an empty OJ-concentrate can), while it resolves perfectly through my speakers. Not sure if that's a limitation of the headphones, though I do see that they're capable of better sound than I was getting.

The Portal Panache is new to me and looks like a great product. That might be a good future upgrade for me. For now, it's a little out of my reach. I've read a bit on Head-Fi but haven't joined yet, so I appreciate everyone's input here. Still don't know if I should put the Schiit or headphones on the list first (ideally I'll eventually do both)--but I'm open to further suggestions! Admittedly I'm still not a huge headphone fan, as listening fatigue sets in pretty fast for me even at low volumes, but they're good for keeping peace with the neighbors.
I just upgraded from a Maverick Audio D1 DAC/Headphone amp driving Sennheiser HD 545's to a Peachtree Audio MusicBox driving Sennheiser HD 598's.My headphone usage has gone from must use so as to not disturb anyone during the wee hours of the mourning to can't wait to throw them on & rock out so I would answer a resounding YES to your question!
In addition to head-fi.org check out headphoneinfo.com,headfonia.com,onheadphones.com & headphonetalks.com...
Well, folks, new headphones it is. The HD555 are excellent for the money, but to start with I think I'm to going sell them and look for a used pair of AKG K702. I'll buy an amp later, when my budget allows. The Schiit Asgard is still at the top of my list, though I am concerned by the 6 Moons review, which characterizes it as being a bit lean--and lean is the opposite of what I want in a headphone amp. Any other suggestions around $250? Thanks for the input, all.
Headphone amps are a lot of fun. I WOO Tube headphone amp. You can play with the tube amp by "rolling tubes" which is trying out different types of tubes.

My headphone amp is in the office and part of a computer audio system.
I don't own a headphone amp but if you feel like practicing your soldering skills the Bottlehead Crack gets great reviews.