The Pro 900 is better than the M50, but for the e7 I prefer the M50 simply because it is a shade easier to drive and it just pairs with the e7 remarkably for a cheap set up. The M50 has just as much bass IMO, just not the same quality as the Pro, the highs to me, are the biggest improvement on the Pro compared to the M50. But also consider your comparing a $120 headphone to a $400 headphone. I think the Pro is a remarkable headphone, but at the same time I also think some people blow it out of proportion. I think this headphone sometimes gets more credit than it should. With that said, I would 100% recommend it to any bass lover, and particularly anyone that listens to a lot of trance, dubstep, etc. The Pro 900 is not an "all around" headphone, if you have specific needs, this headphone will do a great job. To my ears the M50 and the DT 990 has just as much bass impact as the Pro, just all have different quality, tone, and speed.
Also if anyone is looking to purchase this headphone and have never saw it in person, do not be fooled, the Pro 900 is a lot bigger than some pics online. Pictures over the internet look deceiving, this headphone to me was a lot bigger when it arrived at my house. It's not monstrous, but just surprised me initially.