The Q701 is a good upgrade over an M50. Don't worry about the FiiO products, either. No, they aren't the best, but they aren't expected to be at their price. There is a purpose out there that they serve and they do it pretty well. Consider them a gateway drug to bigger and better things.
Hah, if my wife knew that those ATH M50s she got me were a gateway drug, I think she would have reconsidered!
I'm wearing the HiFi-400 right now, been testing it today. At first I wasn't sure what to think, just because I'm used to the closed headphones and I couldn't seem to get the volume high enough. But, I had some weird sound settings on my PC, and after I fixed that they sound amazing. There are still some songs that are too quiet, for some reason, but I'll see how they do with an amp. I never understood what people meant when they say that a headphone is "precise" or "detailed" until I tried these.
Not that it matters for sound, but from an aesthetic standpoint, the Hifi cans look ridiculous. My wife walked into my office, froze, and laughed when she saw them. Then, I had her try them. She typically dismisses my new gadgets immediately, but she wore them for a few full songs.
I knew the Q701s would be much better looking, but I was interested in the sound from the HiFi's planar tech. After having them for only a day, I'm actually growing to like the ridiculous look of it, probably just because they sound so good.
I agree 100% so far with what MLE said about the HiFi-400, especially with the bass. I listened to a few songs that I remembered had more bass with my M50s, and I was slightly disappointed. But, I tried a handful of more bass-heavy songs, and the difference was huge. Just like MLE said, if there is bass, you're going to hear it.
I've been listening to the M50s for almost 2 years, and while I still like them, especially for the price, it seems like when you're used to a particular sound that it takes some time for your ears to adjust. It probably took 10-20 minutes for me to adjust to the open headphone "atmosphere".
I get the Q701 today and the FiiO E10, I'll give a quick review once I've tried them both with the amp and I decide which headphone that I'll keep.