Ryokan
Headphoneus Supremus
Impossible to answer this question mate.
How much does the sound improvement worth, that's something you have to decide for yourself.
For me, M9 is significantly more mature and balanced than Andromeda (2019 version is what i had briefly before), and since it fits better as well, price difference doesn't really matter, i would not keep Andromeda even if i get it for free.
At the same time M9 just sounds boring and dull for some people, and they have much more fun with Andromeda.
Does that mean the M9 is similar to the W80's? I find Andromeda are ok for fit but they soon work loose when moving about.