I'm going to throw in a real curveball and suggest the Sennheiser HD565 as one of the most underrated 'vintage' headphones available.
My dailys are HD600/650 and LCD2 Fazors but when I feel in the mood for a comfortable and fun listening experience I reach for the 565's.
No silly EQ necessary, they have a beautiful, engaging SS all their own, but you'll miss some bass coming from the M50's - I have 2 pairs of those too.
HP snobs will baulk at them but they fit your budget, if you can find and then restore a pair! 150-Ohm so that may be a fly in the ointment.
Refurbing them is easy as they are relatively easy to dismantle, refurb and/or replace the pads.
I have 11 pairs of them (not for sale). I don't know why I have that many pairs exactly, I just do. They form part of an overall collection of the 'Tullamore' era of the HD*** range which I have become a bit obsessed with.