If you have 400$ you should get the best headphones you can get. the sennheiser 58x and the low level beyerdynamics are ok but their not gonna be much better than your m50x. get out of the 150$ price bracket and get some headphones that will blow your mind. I recommend the Sivga P2. one of the best planar’s in existence. its 400$ or get the hifiman Sundara at 350$ another awesome Planar. The Sivga has a really nice V shape sound bass and treble a little elevated, the Sundara has a nice W shape, treble, midrange and bass slightly elevated. these are among the most exciting headphones around. with incredible sound that completely destroys the 58x and dt770. also both very durable
you can save up for a amp later, headphones are the most important part.