Which headphones??

For soundstage and imaging, you may be better off with open backed headphones. Soundstaging and imaging are precisely what you tend to lose on a closed headphone.

Around $200-300, you might want to try the modded Fostex headphones by Mad Dog. They get excellent reviews, and are closed backed.

Senn Headphones tend to have a pretty relaxed sound in my experiences, and they are also among the most comfortable brands I have tried.

As for higher-end headphones, I'd recommend taking advantage of The Cable Company's lending library. I think that this service is only for headphones above a certain price point. But at these higher price points, you are getting some pretty spectacular sound. Though you will be limited by the zDAC on some of these higher end headphones. I am a huge fan of the HiFiMan HE-5LEs that I have, and other headphones in their line are also excellent (though I'd stick to their orthos).

As with most things audio: audition, audition, audition.

Be sure to consider using a separate headphone amp which will drive your phones. This can make a huge difference. I'm using the Senn 598s and a small Fournier tubed amp (costs about $175 and is sold on E-Bay) in my TV room. Nice relaxed sound.

I use Senn 650's and a Woo W3 amp in my main system. More resolution and extension yet balanced slightly on the warm side. Costs 150% more though but it sounds quite a bit better in some respects than my main system, especially in clarity in the mids and highs.

I would suggest though that you do not expect a soundstage even remotely similar to what you get with a properly set up 2 channel system. Despite all of the sales pitch about imaging and headphones you will read, headphones really don't come close.

The 598s are supposed to be v. close to the 600s but that price seems a little high.

I have a dedicated headphone amp from a now infamous builder Singlepower. It is an OTL and sounds great, it is a pity that the builder/owner ripped a number of people off and disapeared from the scene. The Headphones I use with it is the AKG 701 which is detailed and spacious sounding. I recommend them as a good sound stager.