If I had to downsize a rig with a budget of $500 I think I'd do the following, all of which would be used.
Source:
Definitely go the computer as source model, several good suggestions for this so far with the 0404USB and M-Audio Transit. Both available used with some regularity and I have to admit, at work I as pretty happy with the AV710 - that might not be an option for you.
Cables:
Absolutely go the DIY route here, you can find good (actually great) cables for cheaper than cheap. There's even a few idiots who if asked nicely will make a set of cables for the cost of parts.
Amp:
Once again, I'd think the DIY used market would be the place to go for any bargain hunter. You can probably find a "home-type" PIMETA in the range of $100 to $150 and it'll just about as good as most commercial ~$400 amps. But since at this point you've spent less than half your budget I'd suggest splurging a bit and trying to get a used PPA. You might find either a V1 or possibly a V2 in the range of $200 to $250, that's an absurd amp for that kind of money. There's also the outside chance that you could find an M3 in the upper end of that range, but those are more rare from what I see. Also, do yourself a favor, if you're looking at a used DIY amp research the builder. I'm starting to lose count of the number of amps that I've had to rebuild for other people as a result of abhorrent build quality.
Cans:
Best value for the dollar, assuming you don't universally dismiss them, is the HD580. Still available for ~$125 you could pick up a HD650 cable and 600 grills if you really wanted to.
So my math says that works out to be about $150 + $250 (max) + $150, right at your target budget and there are ways to shave a bit here and there if you really wanted too.