Oh no, I'm not saying you should sell them. (in fact, since we're talking about the HD 650, I'd even tell you to keep them like your life depended on it)
I'm just wondering why you wanted a portable amp specifically for them.
With that said, I think you'll be tempted by someone to look at the Fiio E12, but... personally, I don't think the E12 is a good pairing with the HD 650.
One very good option for under $200 is the Objective 2. If you're in Europe, I think you can buy one from Epiphany Acoustics.
Or if you're simply looking for the absolute best you can grab for $500, I think the Portaphile 627 (or the new 627X) would be worth your while.
Edit: and of course, since I own one, I guess I'll throw the Leckerton UHA-6S Mk.II up for your consideration. You can roll the opamp if you don't like the sound signature, or pop a pair of OPA627 in and have some interesting comparisons with the Portaphile 627. It's only $300, so you're saving quite a bit from the Portaphile.