The HE-400 is the best sub $500 headphones on the market right now by far! Make sure to buy the velour pads with them as well as they make a huge improvement over the stock pleathers.
In regards to amps; I own the O2 amplifier from JDS Labs, and it is probably the best one you can get for these headphones, but they do cost about $150. If you are on a tight budget and you have access to a soldering iron, you can buy the Cmoy amp from JDS Labs and get a TL2426 as a free sample from ti.com... this will give you enough power to drive these suckers well and I can attest it sounds pretty good. This will run you about $50
I own the HiFiMan travel case as well, and I would recommend NOT buying it. The HE-400's will not fit in the case while the connectors are plugged in, and the sockets on the headphone will break if you keep removing them each time you put your headphones away. Furthermore, the styrofoam material on the inside of the case that keeps the headphones from moving around is two pieces which are cut or aligned with precision, so the HE-400 will get stuck easily when trying to pull them out of the case.
My last piece of advice is to purchase the headphones directly from HiFiMan's online store. There was recently a recall on the newest revision; HiFiMan's store removed the bad sets from their inventory, but if you purchase from somewhere else, you may get the crappy revision if that's what they have in stock. Don't worry though, shipping is very cheap and quick.