Man i was working on some detailed write up, I chose a list of songs to A/B between the Momentum and HP50, had the notepad for taking notes and everything. I got half way through and realized... what's the point?
In short, the HP50 just does everything better. The biggest issues I have with the Momentum are:
1. Bass is nice, but it delivers more than the song is calling for; it doesn't get out of the way fast enough (decay), while encroaching on the mids. It just spills over and is sloppy.
2. Mids are congested. More on this below in the song example.
3. Highs get swallowed by the mids at times. To be fair, there are instances where the Senns' have that nicer bit of sparkle to the vocals, but the problem is that the vocalist is in the wrong place. The image I always get is a person who's stuck halfway in to a pit of quicksand (mids) fighting to be heard. You sound nice but you're stuck in some muddy shit bro. Put on the HP50, and the dude pulled himself out of the muck In short, the vocals should not have to contest with the mids.
Take a song such as
James Blake - A Case Of You. When I listen to this one in the HP50, I close my eyes and I can just see it perfectly: James is sitting at the piano, playing while singing into a microphone. And his voice is actually above the piano keys because of the way he's sitting and singing up into the mic. With the Momentum, his vocals and the piano riffs are closer to the same level, like they're being crammed together in a tighter space. Neither element gets to shine on its own, and the song loses the ethereal sense of air between each component.
Soundstage, imaging, etc: The best way I can describe the overall effect of the Senns, is that they create a "wall of sound". I find myself confused, unable to pick out placement of the different elements where they're supposed to be. Ya know, I heard this "wall of sound" complaint a couple times before I bought them, but I was swayed by the glowing praise some unnamed headphone review sites were giving the cans. On the other hand, the staging of the HP50s is just natural to my ears. They don't really place things, they simply present the arrangement as it's meant to be. I've had some surreal moments where I close my eyes and it's like virtual reality, the way I can see and feel where everything is. Like in the James Black example above, I started to feel exactly how far from his mouth the piano keys were. Since i bought them, I haven't had one of these moments with the Momentum; everything is just front and center, boring.
For the rest of my fellow Hip-Hop heads: The two headphones might be equal, to my ears. No, the HP50 does boom not like the Momentum. What I came to realize is that the HP50's bass extends much further than the Momentum (seriously the sub bass is so good), and it's much cleaner, honest bass. If the song booms, the HP50 booms, but if the bass is more reserved, you'll hear it that way. In this genre, the wall of sound image is strong with the Sennheisers. Rappers are smothered in the Senn mids. It's really syrupy.
Conclusion: NAD Viso HP50 is clearly the superior set, and is for hose who really want to be there while they listen. The staging, layering, imagery are the best I've heard from a closed set, in the $250-$300 price range. Without a doubt they are worth the $50 premium over the Momentum. There were times where the Senns were a bit more "lively", but I was quickly distracted by its presentation failing in the technicalities.
So that's it. The Sennheiser was a large upgrade over the AiAIAI TMA-1, and I feel going from them to the HP50 was a similar jump in quality.
Questions? Comments? Anything I missed? Want to call me crazy? Want to buy my Momentum? Feel free to respond.
p.s. - I didn't get into the whole Room Feel™ technology, what is there to say. The HP50 is a better open headphone than my HiFiMan HE-300. Kind of weird to say that about closed-back headphones.