Thanks for reading and for the question! This one might be a little bit tough for me as I don’t have experience with a wide swathe of earphones, but I’ll do my best.
In over-ear headphones, I think the $99 US Pioneer HRM-5 is probably the best M40X/M50X competitor…but it’s not really any more manageable.
If you want something that also has some nice gaming headset capabilities, then the newer $99 Logitech G433 has a very balanced sound, removable mic and cable, and an extra cable for use with mobile devices. It’s a bit more portable than any of the studio options.
The flattest in-ear headphone I’ve ever used is, believe it or not, the Bose SoundTrue Ultra. It’s a fully sealing in-ear headphone but the tips are still pretty comfy. When it launched it was $149 US, but now it goes for around $129 US. So that puts its price right in the middle of the M40X and M50X.
As far as sound signature goes, Audio-Technica has a tendency to tune everything to their very specific “house sound,” even the stuff they mark as being a little more bass-heavy. So if you go with one of their in-ear models, it’ll probably have a similar sound signature, even though it won’t be a one-to-one match because of the different design aspects of over-ear vs in-ear models.
I hope that provides you a little help and at least sets you down the right path. Sorry I’m not more of an earphones guy! 99 percent of my listening time is with over-ear headphones. Thanks again for reading.