I recall reading somewhere that non parallel walls help with better acoustical properties.
Maybe a bit too late to consider this but it’s possible to do this to an extent while plastering?
I did come across this:
Short Answer: No — and here’s why. There’s a long-standing myth in acoustics: If you build your studio with slanted walls, you’ll eliminate room modes. Let’s be clear — that’s not how sound works. Yes, non-parallel walls can influence the acoustic behavior of a space. But they don’t make modes...
www.kissyourears.com
Given that acoustic properties of a listening room is considered the one of most important factor that impacts the reproduction of playback sound this is an exciting project. Please share regular updates on your design choices and progress.
This might be of some use:
en.wikipedia.org