The wing system on the audio technica headphones is the best "head band" in the headphone industry for me. I hate headbands that rest on my head, I never find them comfortable. The wings on the A900X are beyond comfortable for me, I can wear these things for 6-8 hours and they feel amazing.
I'm not sure how anyone can possibly feel that the A900X has more bass then the M50's, simply not true. The A900X's have a better quality low end but they certainly do not produce more bass then the M50. The M50's have a major overload of bass that bleeds into the mids and sounds nowhere near as good as the A900X.
As for the K550, they are a nice headphone, I'd put them on par with the A900X, sonically I thought they had similar characteristics. I was considering buying the 550's when I got my A900X, I didn't find them as comfortable and I also had issues with getting the pads to seal properly for some reason. I felt like I kept having to adjust the pads and push on them to get them to seal. Not sure if it was because of my ears of head shape or what, but I read a couple of comments on here where people also mentioned it. I've also never noticed any sibilant highs in either of them.
My problem with the A900x headband is the fact that the headphone itself weighs more than the force needed to push through the wings. The claming force keeps them on the head and clamps down more toward the bottom of the ear and upper jaw/neck. Very plasticy build as well. The k550 is mostly aluminum.
The M50 is certainly not a bass heavy can. It has nowhere near the amount of bass that something like the A900x or DT990 or even the K550 for that matter. I own or owned all those as well as several more neutral headphoes and know what they sound like in comparison.