The EY500 are the correct equivalent tube to the 6D22S, just keep in mind they are almost a full inch taller than 6D22S and with the power supply chassis of my 445, they do not fit, but I run it topless. Do this at your own risk as it is high voltage. There are lots of EY500s on ebay for ~5$ each, just make sure to get a matched pair. I've had good luck with NOS GE EY500 which Craig recommended, they sound better than the 6D22S having a more open soundstage and higher quality low end. EDIT: Forgot the ZD does not have internal rect tubes, I guess height isn't an issue so long as you have long enough leads for them.