To UNSOUND: To answer your question regarding "what a large sphere shaped room would sound like, if you sat in the middle of it", the answer is that it would sound as worse as it possibly could!!!! The reason? Well my experience with acoustics suggests that you'd have so much reflected sound focused right at your ears, that you'd the sound would sound like you're head waz inside an over amplified megaphone!...magnified! ALWAYS avoid "concave" surfaces in your listening room, as they "focus" the sound right back at your ears like a lense, making for very unatural sound, or the antithisis of open, uncongested, even sound.
For reference, read F. Alton Everest's book "Master Handbook of Acoustics" (any edition).,