IMO the perfect alternative to the M50 is the Koss Pro DJ 100. Before I found out about this headphone I had the SRH-840 and side-graded to the M50 due to comfort issues.
Once I found the DJ100 I was hooked and I've been raving about it ever since. I think I've been through at least 5 pairs of them by now! No, not due to defects!
Sure, they're $80, but I'm sure $10 more won't hurt.
The DJ100 has more forward mids than the M50, maybe 5% less bass, but it looks like in graphs they may have more extended sub-bass. They are NOT bass heavy, but do have a slight bass boost. The KRKs make them look like bass monsters!
To me they don't have so much bass than it bleeds into the mids and ruins the sound. Pretty much the most I can take. Best thing about the DJ100 is how well they do female vocals. Has the KRKs, HD-598 and everything else beat.
Some might be confused about the DJ100 because they don't seem to have a massive mid-bass hump. Never figured out why some people say they lack bass. They're almost bass heavy!
They do have less treble than the M50, but to my ears are much less fatiguing despite the more forward mids. I actually use mine with the M50 pads.
I normally don't like to suggest them without an amp, but they do work and sound great from a Sansa Clip! Yes, that tiny thing drivers some headphones better than even players much larger!
I use mine with Rockbox and use nothing but FLAC files on a 32gb Micro SD card.
DJ100 is also often on sale for $50. It's not as neutral as the KRKs, but a little more fun. Not quite as clear as the KRK KNS-8400, but the DJ100's level of detail isn't too far behind.
The Denon D1001/Creative Aurvana Live is also OK. Sound clarity on it is not bad.
For a more V-shaped signature you can try the Superlux HD-668B. It seems it's focus is on treble and bass. It's a very clear sounding headphone and is much better than most of the other garbage you can get for under $80.
I'm not a fan of it's signature, but it's one I can suggest. It's mids are kind of recessed.
BTW I comparing all these to the newer white boxed M50. The older pair I also had wasn't as good and had quite bloated bass and was not even remotely treble happy.
EDIT: I just read your comment about wanting slightly less bass than the M50. Then to me the DJ100 is perfect.
I also don't find the KRK KNS-6400 to be bass light at all, but pretty close.
I also think the Sony MDR-7506 isn't too bad. It's cheap and not quite as good as the KRKs, but has more mid-bass and are more portable.
The XB500 also CAN be good, but ONLY if you EQ the heck out of it. If you don't, it's unlistenable to ME. Without EQ it has a severely muffled sound (like the Kicker). Way too much bass bleed.
If you have a Best Buy in your area, you can buy the DJ100 (check stock online) and return them if you don't like them. The UR-55 isn't bad, but sound clarity is just average. It's also not that comfortable, but is more bass heavy than the DJ100.
Fans of the Porta Pro might like it. It actually has good mids, but rolled off treble.
If you're willing to give up some bass I love the Beyerdynamic DT-235. It definitely has no lack of treble. It's only $60!
