It probably sounded similar because you had a bad source AKA your computer or low-quality music files such as MP3s that are compressed. Once you spend some money on a dedicated amp and dac (doesn't even have to be that much, something from FiiO will already give you a big improvement) you will start to notice the difference.

will i agree on the quality, it should been better than IE80 at least... yet the IE80 made me enjoy more, i had both, for the system i got from Fiio to WA7 fireflies and i gave them the amplification justice, and i say it wasn't worth the price what so ever, for a 350$ i could many better options soubd/value wise like a silicon CIEM from the custom art and top this sound quality 10 times, considering  i want HQ portable headphone for this price the momentum is just way too over rated, 
 
note i love senn products a lot and HD650 , IE80 still one of my top HQ headphones. 

Source is probably not the problem, as the computer got the newest Realtek audio chip and I'm listening to 320 kbps on Spotify. The box even states that these are "made for ipod, ipad, iphone", and then I atleast expect it will sound good on my iPad. Amp should not be required as these are easily drivable.

its still a nice portable headphone but not for the price, and yes they won't sound bad without an amp, but i am sure a portable amp will tend to improve the sound a lot, like pico slim amp which tend to make the sound way more magical <3 

Trust me, the default audio is pretty crappy. The HD600 and HE-400 sounds better out of my Fiio X3 than my laptop. They sound a lot better even just out of the Modi. Even so, if you don't like them, you don't like them. I can't imagine they are worse than the M50 but who knows. I do think they're overpriced.

There are no bad headphones from a certain Price Level on, but all depends on your personal taste and expectations. Price depending also on where the headphone is build, if you device is build in the US, EU, and or Japan it will be much more expensive than when made in China, due to the salaries and social Situation, wich does not mean that China build headphones are of a lesser Quality, but for myself I prefere a fabrication where I am sure there are social Standards like we wish them for ourselves.The Momentums are hyped everywhere, but only trust your own ears, and don´t expect that the double Price doesn´t mean it is double as good, because of reasons I explained before.