feel that t is not a good idea to switch the negative power like you ask abot.
A: is not the common practice.
B: if the B+ (12 VC) and the PA + are different voltags, there is, IMO, a voltage differential between the B+ an PA +, so would possibly be some power feeding a part of the unit. That might or might not be a problem, and possible drain if running from batteries.
Old School Electronics