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
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.
The K702 are the best bargain in high-end headphones. They are open and clear and fast and musical and to my ears simply blow away anything in their price class. Yes, it took over 300 hours of burn in to fully tame the initial high frequency brightness, but after that you're in heaven. You'd have to spend way over $1000 to get sound this good, and, they are so comfortable I can wear them for hours.

I feed them from a Ray Samuels Audio tube headphone amp called The Raptor, which costs $1200 and is just magic with the K702s... Close your eyes and you're gone! Ray has a SS amp that is in your budget, but I highly recommend you hear them with tubes before deciding. Buy used and you'll be surprised how much you can get for your money.

Enjoy,
Alón