The M20X would be slight sound upgrade, but maybe not as much of one as you’d expect for the price hike. Especially if you love bass. You can check out good comparisons of sound measurements over at Rtings. Here’s a comparison of their measurements for the M20X and the UR20. The M20X is closer to flat in the midrange, but they don’t have nearly as much room for your ears in the pads as the UR20’s do, which will bug most users.
Headphones - 1.2 - Graph
Wireless Earbuds Wireless Bluetooth Running Gaming Noise Cancelling Earbuds And In-Ears Xbox One PS4 True Wireless…
As far as something new, it really depends on what your personal tastes are. Never tried an open back? Go for the Koss PortaPro! Want something closed and don’t mind a smaller headphone? Try the Creative Aurvana Live! Those are my go-to recommendations for quality audio at or around that price.
Apart from that I’d almost recommend saving up to the $100 level. There, a whole world opens up. Sony’s MDR 7506/V6, the Audio-Technica M40X, Pioneer HRM5, Beyerdynamic DT240, Sennheiser HD569…the list of quality products at $100 is just absurd.
But it’s ultimately down to your personal tastes and preferences. I can only share mine!