Frankly the HD681 EVO walks over the M50/AD700X in my opinion (I have a white pair, they're quite a high performing headphone, regardless of cost).
At the $150 range, I'm going to say that you're not going to get higher quality sound. You will get higher quality build and features. But the audio quality is going to be very minimal in change at this cost range.
Do you want to maintain the bassiness of the HD681? Or do you want something more neutral? At...