A good DAC + a good amp will cost you around 200,- used. I can recommend the FiiO E09K, I got mine for 80 Euro. The successor to it is the K5 which is supposed to be slightly cleaner sounding but a bit less powerful. As a DAC you can go for an ODAC or something in that price range. My Musical Fidelity VDAC II is also pretty good.
I hear the term "Trebble Cannon" attached to the DT990s, which are often highly recommended. Is it "Better" than an M50x? I dunno, I think they're more comfortable?
The 250 ohm DT990 with a DAC/AMP would be interesting, perhaps?
Also we (Antlion Audio, the ModMic guys at which I work) just reviewed the HIFIMAN Edition S, which is a really interesting pair at 250.
They bring some interesting options to the table since you can use them in both open and closed back, sounds great, but again, I feel it will largely be a sideways move from your M50s.
Sadly, at ~250-350 USD (which is probably about what you paid for the M50s) you're starting to hit the diminishing returns where spending more produces a very small gain. I'd agree with everyone else that a DAC may provide more quality than anything else, as if you're using on board audio drivers or even a soundcard you may be getting more white noise and interference than you think.
So ... TL;DR, start with a DAC and see if you can tell the difference and/or get some DT990s or others and see if they are simply more comfortable. I like my M50s, but they honestly hurt my ears to wear for extended periods of time (which was why I upgraded to the R70s - which I guess also fall into this price range, albeit just barely)
I really like grado cans myself for that price point you could get the SR 325E or go with a SR80 or SR125 driver and then swap out for some wood cups (will change the sound a bit depending on what you get) tends to make them sound more like the RS line
The grados are a really love it or hate it sound. I would describe them as flat and accurate with a warmth in the high end. I would find you a freq chart to help but the headphone rooms' website who does most of that for the cans on the market is down for maintenance at the moment.
I personally really like DIY or mod can setups. so something like these work well for a grado like sound.
The Ypsilon Headphone Drivers and would Cups you could get from http://www.ellevenacoustica.com/products apparently are pretty darn good. ($90 for the pair of drivers and 76 or 102 bucks depending on the wood cups you like but there are a lot for guys amking these wood grado like cups and doing the grado mods and driver swaps.) Or turblent X drivers work great too or the sennheiser pxII drivers too apparently.