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!