[marek17]

My brother wants to buy Headphones but does not know which ones? Best Sound/soundstage (T51p?)
My selection:
Beyerdynamic DT 440
beyer DTX 350 p
Beyer T51p
Beyer DT1350 (or used)
NAD Viso HP50
SONY MDR-1A /1R
AudioTechnica AV500
AT M50x
AKG Y50
AKG k142hd
AKG K550MKII

added:
Sennheiser Momentum2
Oppo PM-2 (price limit)

 

[buke9]

My brother wants to buy Headphones but does not know which ones? Best Sound/soundstage (T51p?)
My selection:
Beyerdynamic DT 440
beyer DTX 350 p
Beyer T51p
Beyer DT1350 (or used)
NAD Viso HP50
SONY MDR-1A /1R
AudioTechnica AV500
AT M50x
AKG Y50
AKG k142hd
AKG K550MKII

Out of the ones on you list only have heard or owned the Nad's . I sent them back only because of fitting problem. The bottom of the cup did seal for me I don't have a big head or anything and most of the time I use very little of adjustment on the headband of my other cans. They sound very good. You also might put Senn Momentum 2's on the list and for a bit more the Oppo PM -3 's are very good also.

 

[GGXtreme]

Would you be able to tell us anything about listening preferences? Music/movies/games? What genres? Does it need to be closed-back? Etc.

If you're considering the K550 and M50x I'd say as far as "sound/soundstage" is concerned you may want to consider the SoundMagic HP150 and the AudioTechnica ATH-A900X. The K550 is very particular on fit to get a good sound (I wasn't able to unless I pressed against the pads and I have a pretty average head size), the M50x doesn't quite have the same bang for buck imo.

The HP150 is a bit analytical and bright with enhanced sub-bass (balanced/slightly U-shaped sound), very detailed and dynamic, wide soundstage and versatile as far as genre preference. The A900X is much more laid back with a natural presentation, a more airy 3D soundstage and rolled off sub-bass and treble (a bit warmer, mid-bass emphasis), good for stuff like jazz but also does well with movies and other genres if you don't crave a lot of sub-bass.

 

[PinkyPowers]

For naturalness, air, comfort and soundstage, it's hard to beat the Sennheiser Momentum 2.0 Over-Ear.