I know folks with vintage amps that run high plate/screen voltages that have been using the Shuguang's. You can buy them cheap from RS :
I've used them in the past in a Trace Elliot Speedtwin and that runs about 500-510. It also has a standard Marshall 100W replacement OT as the tech who repaired it is a hack. It really should have a 2k-2.2k primary for that kind voltage.
Screen voltage rating do get vastly abused in most amps. Said speedtwin also runs 6L6's and they are only rated for 450V on the screens and they see 500! Same is true of the SLO.