EY500 help please? Phil, others who have used it?

[Nick]

Surely swtitching noise or whatever else silicon be it fet bpt or diode does which makes sound worse than when a valve is employed instead is best not present at all.

Maybe, but the normal cause of diode switching noise is the internal capacitance causing a spike of reverse conductance when reverse biased. Using in series with a thermionic diode which doesn't have the reverse current, stops the reverse current in the diode from happening so removing that source of noise from the silicon diode. And the soft curve at low voltage and current for the valve diode stops the sharp cut-off of the silicon diode from causing ringing in the transformer.