Try some Rothwell attenuators plugged in at the power amp end.
I had the same problem as you. I have a VTL valve pre and XTC transistor power amp and the noise floor was high.
I thought about replacing the pre but since I put the attenuators in it sounds so much better that I am now happy with my set up.
Saved me a packet.
Don't know why your valves are failing though. The 60's are lovely speakers.
Thorens TD550, Ortofon 12", Ortofon Jubilee or Koetsu Black; Rothwell MCL SUT, VTL 2.5 pre, XTC POW 1 power; Naim CD5x; KEF Reference 3.2's; Van den Hul interconnects and speaker cables.