I've been able to go with Svetlana EL-509's... Keep the Deadkey at 2 Watts swinging NO MORE than 5 Watts RMS... Or 15 Watts PEP .... And run the Amplifier Hot and heavy for 8 months now with no trouble...
The Amp still yields 650 watts RMS/ 1300 watts PEP and is very clean and great sounding when TUNED TO AUDIO... NOT by the way the book tells you!!!
When installing New tubes... Let them cook for 48 hours on standby, Run them on Low power for a week and than Medium for a couple days... Than run High power as needed...
Took a couple sets of tubes and some learning curves with this Amp... Follow the prescribed method above and you will get plenty of enjoyment on your radio... And with your KLV...
Here's a link for the tubes:
Svetlana EL-509 Heavy Duty Premium Tubes
I swear by them...