Hi Steven
MR65b, MR67 and MR71 all had selenium rectifiers.
Earlier MR65s had tube rectifiers but were essentially mono tuners with a bolt in MPX decoder .... this is the one I have.
This MPX decoder was rushed into production when stereo broadcasting started unfortunately had a transistor to flip phase
in 1 chanel so the path for L & R channel are not perfectly symmetrical.
I guess one could get an external tube mpx adapter like the Scott 335 using the mpx(mono) out from the tuner.
OR ...........................
Look out for a Scott 310e