If it's solid state, then it's not a tube. There are replacement solid state rectifiers that plug into the tube socket, though.
When I replaced the rectifier tube in my 200S with diodes, I noticed a slight increase in power. At least I think I did. It just seemed a little more solid when pushed. Probably due to eliminating the power supply sag. However, I can't say that it's the diodes that did it, because I also replaced the caps at the same time.