I'm leaving it for others to try to resolve your issue, I'm simply gonna address your thought of replacing the DT770 with the ATH-M50X.........DON'T!!! The M50X is simply overpriced for what it brings to the table. I'd auditioned the M50X a while back, and I can only say it sounded rather 'meh'. I can recall that mids was horribly muddied by the bloated bass......and wondering why peeps buy it in the first place.
Oh yeah, if you like the signature sound of the Beyerdynamic DT line, perhaps you ought to look at the 'Premium' line instead of the 'Pro'. I have a DT990/600 Premium and it has much less clamping force compared to the DT770 Pro......I say this with the utmost confidence because I'm an owner and frequesnt user of both! Just a general suggestion, wait for others to reply....
Its best to find the best voltage settings on manual for your self, though 1.36v is good for finding your sweet spot, so what i mean is this:
set vcore voltage to manual and set it to 1.36v and play with the frequency and voltages based on your needs and what you find to be stable, if you want to play it safe don't go above 1.4v, when doing stress tests use hwmonitor to see temps just in case. Btw if your satisfied with your current OC you can just leave it alone and manually decrease the voltage until you find instability. No sense running more voltage then you have to in the CPU, but another personal preference is when i do find the minimum stable value to add an extra +0.02 or +0.03v to it for proper performance, for instance even if my G3258 was stable at 1.32v it was not able to use its full potential (had ocasional stutters), i upped it to 1.35v and it runs perfectly.
Also after your done with the cpu if you want you could tighten the memory timings a bit as well.
As for a general guide line people usually use prime95, OCCT or intel burn test for 2 to 24 hours to check for stability ( the longer the better is the usual standard). Personally i prefer to do it differently: