Stefano,
OK on the 30:1 transformer. So this is a 60hz power transformer being used for audio... interesting. I know some (including myself) who have used them for plate modulated tube service, but none yet for solid state audio. Have you swept the rig for audio and checked results? That will tell the story if you might need some feedback compensation to tailor the iron. Otherwise, consider some negative feedback from the mod output to a low level audio stage to clean it up further. With a big rig like this, your side products will need to be 10db cleaner than the average AM station on the air or they will complain. You know how that goes...
T