>>391164727
Why you reencoding twice, are you retarded?
What is this fucking line?
>ffmpeg -i 4ch.webm -c:v vp9 -b:v 1600k -an ready.webm
And it wouldn't even work because "-c:v vp9" is not a valid command.
If you want to strip the autid just do "-vcodec copy -an", you monkey. By reencoding twice you are just multiplying the artifacts. 

Also, "-speed 1" in first pass is useless, it's changed from vp8. Second pass should have "-speed 0" too.
"-time-column 6" decreases the quality.
And you don't even have "-row-mt 1". Oh my god. The more I look at your shit the worse it becomes.