The PSB M4U1 sound awesome. Make sure you get them under warranty, there are a few build-quality concerns to worry about beyond a half-year or so in, according to many owners, but in my experience there's almost no other closed-backed over-ear pair of cans under 300 dollars with as flat a frequency-response as the M4U 1. And the soundstage and detail knock me right off my feet. You almost definitely can't go wrong with them as long as you, as I said, make sure to get them new and under warranty, preferably from an actual high-end audio store somewhere. They also provide pretty good isolation. You can get them new online (Amazon, for example) for 249.99 right now.
There's also the Sony MDR-1A. Wouldn't have any build-quality concerns like the M4U 1, so you could be fine getting them used to save money. But the response isn't quite flat, it definitely has boosted bass and scultped treble, but not to NEARLY the extent that you hear in most cans these days, they still give quite a bit of detail and don't muddy-up any parts of the frequency range by too much. And the soundstage is incredible, nearly as good as the M4U 1.
And lots of people keep telling me that the Audio Technica ATH-MSR7 is great if you don't mind brighter-sounding cans. And the Yamaha HPH MT220 are supposed to be a lesser-known gem in the area of closed-back over-ears.
Edit: Oh wow I should have really finished readint through ALL the posts in the thread up to this point, before posting my reply. My recommendations would be good, if you had the kind of budget that is more typical for someone asking the kind of question you were. But it appears that you actually have a budget on the level of being able to get Summit-Fi level gear, so you can disregard my advice