There are several threads on the best quality recordings of the bass, and many people hold up ECM as a model of excellence in the delicacy and naturalness of the sound. I came across this essay on Eicher and his engineer Kongshaug:
http://fernould.club.fr/ecmvo.html
It has quite a few references to equipment makes and models (mics, reverbs, consoles) and makes some points on mic placement. I came away with the feeling, after reading this article, that natural sound is by no means a natural or simple process.
