Would recommend you look at getting a good Amp first, perhaps a
DAC/Amp combo? There's a lot more you can get out of your M-50s and most good open headphones all-but-require a competent headphone amp.
With that covered, the best bang for your buck open headphones in the low-to-mid tier price range are;
Audio-Technica ATH-AD900s (look for second hand or refurbished between $150-170)
Hifiman HE-400s (obviously the better option, but you'll probably need at least $300 for them used)
Either would make good choices, though the latter certainly much more-so (and indeed, it can compete on a very high level). First you should work on your system IMO.
EDIT: Alternatively there's always Grados, but it sounds like you need something with more bass presence.