JBM: Match loudness for stereo and mono in ParamUpmix mode

Basic info

Bug description

ParamUpmix which operates at 7.1.4 160kbps when rendered to stereo and mono doesn't quite match output levels with other modes in the JBM path. Related to #787 (closed)

Ways to reproduce

(Clear steps or refer to a failing automated test, e.g. with a pipeline link)