Going from M50s to K701s, Q701s, or K702s (sonically all 3 of those AKGs are the same cans) is a huge difference in sound signature. Where M50s are kind of warm and bassy, K701s are bright and a little edgy. They have bass but it isn't as "forward" as the Audiotechnicas and they're more clinical sounding.
I'm going to make a strong recommendation out of left field, check out the HiFiman HE-400s.
They're similar in the bass department to the M50s in terms of amount, but everything else is so much more rounded. It's a fuller, more open sound that's tuned for tonal balance and to have better treble extension, but without being harsh. IMO, these are the best headphones under $300, bar none.
Why are the HiFiman HE-400s so good? I'm glad you asked! ;) One, they aren't a traditional dynamic driver design, they're a planar (or often called an orthodynamic) headphone. Much like the way a flat-panel speaker (ex. Martin Logan and Magnepan) works, they make sound by running current over a very thin membrane and this typically allows for faster dynamic response leading to lower transducer latency and much lower Total Harmonic Distortion.
But you may ask, "isn't this technology usually a lot more expensive?" Yes, it generally is, however, using a new process and more abundant materials, they've been able to drastically lower costs without greatly reducing sound quality. At one time if you wanted planar headphones you had to pony up $2000+ and buy a special electrostatic amplifier to boot, and if you want the very best you still do. (Check out the retail pricing on Stax SR-009 "Earspeakers" sometime, it'll shock the hell out of you.) So the HE-400 is the value option in the market, and they truly are that, and then some, because they flat-out blow the doors off any traditional driver headphones in their price range, IMO. So find a retailer, like Amazon, that allows returns and try them out, you don't have anything to lose. (Not that I think you'd ever want to return them.)
Oh, also go to schiit.com and get a $100 Magni headphone amplifier and hook it up to your Xonar STX, it's the best Benjamin you'll spend in personal audio. Those little suckers are amazing and will cleanly drive just about any headphone out there.