I checked my MA23 again and it does act like a diode so I think it's ok.
It also drops in resistance when heated.
The way it's wired in the circuit I'm thinking it's acting like a thermistor that I've seen in other audio stages.
An MA23 also refers to an NTE109 equivalent (fast switching germanium diode)
there's a data sheet on this page.http://www.weisd.com/test/GenericParts_ ... id1=MA23(B