Hmm, I've never tried the Amperior either, but I've read some reviews saying they are more suitable for bassheads than Momentums, but obviously some of these reviews can be misleading.
K550 and Momentum are both amazing headphones. K550 is more suitable for home use though, and I have open back Q 701 for that. As much as AKG promote them as somewhat portable, they're too big to be truly so. Momentums are much more portable and fit better on my head, K550 is better for people with big heads.I like AKG but Momentum are better "portable, stylish headphones with a sound quality that can satisfy audiophiles".
I agree. the K550 is really big & i wouldn't want to walk around the street with those around my neck. they also have a non-removable super long cord & no hard case. I would say that you can take them places, but you probably would not want to wear them while walking around.
akg did release the k551 with removable shorter cords, which may be a better alternative for people on the go. the giant k550 earcups can be a pro or con depending on how you are using the headphones. the k550 are super comfy & have great sound stage for closed headphones probably due to the size of the earcups, but that makes it awkward to carry around or wear around your neck. the momentum's tiny earcups makes it more of a semi-on-ear design which is great on the go, but may get uncomfortable for some people during long listening sessions.
if you are looking for great neutral closed headphones for home use, the akg k550 is a solid choice, but yes, the momentum is designed for portable use & would be a better portable lifestyle headphone over the akg k550. there are a lot of other closed portable lifestyle headphones nowadays with great sound too tho.