I'll give you my thoughts on this since one year ago I was in a very similar position.
I wanted bass headphones more than anything - I listen to a lot of UK old school dubstep / bass music and other contemporary genres like EDM etc, and so I wanted something that would replicate a speaker system with a big subwoofer. Throughout my journey both experiencing many headphones myself, and doing some serious research on the internet, this is the list of headphones I would view as the top bass headphones that also have good overall fidelity:
Fostex TH-x00
Fostex TH 900 (highest potential if EQd)
Denon AH-D5000/7000
JVC HP DX1000
There are other headphones like the taktion kannons and jvc sz2000, but I believe these headphones are more single handidly focused on bass above all. The headphones in my list also offers varying degrees of audiophile/high end sound going above most normal equipment in the entry level price range. Unfortunately, these headphones are also out of your budget. So then I think you should consider very carefully what matters most to you:
- Big bass
OR
- Everything else
You see, bass heavy headphones almost inevitebly fail pretty hard in other aspects of sound. The most glaring error is often that it muddies up the mids or that tracks get ruined because the bass just overwhelms the sonic landscape.
What I learned, being someone who loves ALL music, is that the bass music, I play on my speakers with sub or in my car. When I sit down with headphones, it is to experience high fidelity. And bass headphones, while capable of doing this, are always going to come short in other areas.
To "get to the point" so to speak, I advice you to think long and hard about how important a lot of bass is to you. Are you planning to sell the 599s? As I remember the m50s, they do have quite a lot of bass. If you get an amplifier, you could get even more I reckon. I generally wouldn't consider them audiophile grade headphones, indeed you can do much better in that regard. But if bass is a priority, they are a decent choice. But again, in my personal experience... Headphones are awesome for everything BUT bass. There are headphones there that do bass really really well... But they are expensive and still they have shortcomings.
