I do not see why you need the drivers to be 50 mm...
bigger drivers does not equal better sound, and by limiting your self to only 50mm drivers you are really only hurting yourself.
I will assume that the fact that you wanted 50mm diameter driver headsets means that you want a big headset, and I will also assume that price is not an issue since you did not specify a budget.
How about the Beyerdynamic MMX 300?
It is very nice, as is most things Beyerdynamic.
If you use the MMX 300, You should probably have a nice Amp and Dac to use, or else you wont get much out of em
If you want something cheaper, the Sennheiser PC360 is very nice too. It is significantly cheaper, and although not as good as the beyers, they are good for the price. It does have an open back design though, so if you play games in a noisy environment, they are not for you.