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
I appreciate the cautions about the AKGs. I am a little worried that I might find them lean or bright, and due to geography I'm not going to be able to audition them before buying. That said, I thought the AKG K501 was a pretty amazing headphone when I owned it. Its treble was more present than Sennheiser, but I wouldn't have called it bright the way I would for something like the Etymotic ER4P (I got rid of those after a month or so). The K501 did need a LOT of break-in, which I understand is true of the K701/2, too.

I know I really liked the HD600 when I owned them--deeper bass than the K501 with a warmer and smoother sound overall. I might just buy another HD600, except for two things. One, used prices for the HD600 have seemed to rise a bit lately, making the K702 the better value. And two, I just plain think the K702 looks cool! Probably the wrong reason to buy headphones unless I listen in front of a mirror, but still...
Watch the prices fluctuate on Amazon. The AKG K702 is going for under $280 right now and I don't think you'll ever see them for less than that. That price will change fast.
I listen to Sennheiser 650's with a seperate headphone amp and it is the best sound I've ever heard. I have a pair of Sen. wireless phones which I use when my wife doesn't want to hear the stereo or tv. These are MUCH less expensive than the 650's, yet unless you A,B them to each other, the wireless phones are very, satisfying. I suspect your headphones will pleasantly surprise you when you get a good headphone amp. Senns are very musical.
I hope you also have loud speakers as well. You know about the dangers of extended headphone listening doctors are warning us about...

I find that with the better rich quality sound of a tube headphone amp allows me to enjoy at a lower volume, which adds to the safety of headphone listening.

However, since we all want to enjoy this sonic heaven all of our lives, I advise all to give up headphone listening, and get a enjoyable loud speaker set up.