I've been using an audio gd dac/headphone amp for a while and like it a lot. Audio gd publishes info about its amp's output power and impedance which is very useful when trying to find an amp for a headphone. In my experience, a lot of manufacturer's fail to give you this information. I bought my audio gd amp/dac on a blow out from a USA distributor a few years ago. It has plenty of juice to drive my Beyers (which are 250 ohm headphones). If you want an audio gd amp/dac now, you'd have to deal directly with the company in China. There are a number of pretty decent headphone amp manufacturers in China that sell direct.
NWAV guy has some very useful technical information about headphone amps and I would do some reading on his website. This is a useful blog on the relationship between headphone and amp impedance:
NwAvGuy: Headphone & Amp Impedance
I found this useful as well:
NwAvGuy: More Power?
It is hard finding objective info on picking out an amp for a pair of headphones. It is really easy to find a lot of subjective opinions on this.
If I were looking for a new headphone, I'd take a good look at Schiit products (no !@#$, that's the name):
Schiit Audio — Headphone amps and DACs, made in USA.. The guys who run the company have an interesting attitude about their products and the prices are pretty reasonable (starting at $100) for an amp made mainly in the US of A (yeah).
Edit: if I had your budget to spend, I'd probably get the Schiit asgard:
http://schiit.com/cart/index.php?main_page=product_info&cPath=0&products_id=1. You can e-mail the company for more info; they're really good about answering e-mails. The magni for $100 might be a good choice as well,
http://schiit.com/schiit-faq/about-magni/
