sound: same SS, M4U has a bigger sound stage (very slight) and seems to have better detail retrieval when using HM-901 as source... In a way, I like the M4U 1 better because music sounds more engaging...
Please take with grain of salt bcos I only A/B them for one hour and using one portable source...
comfort: same
build quality: same plastic-ish but sturdy
M4U 1 design is a bit "unique" because from the look of it, you would not expect them as "audiophile neutral" sounding phones.....
The reason of getting HP50 is to get that M4U 1 sound on the go.... but the headband really sticks out too much and looks weird on the head... (just my personal feelings).
I will send them for balance mod and upgrade better cable after the holidays.... (not sure if it is possible yet, fingers crossed) I am hoping it will become less "polite" and more engaging...
Are the HP50 now available in HK?