Larry On my amps I can bias each tube so matching is not that critical. I bias each tube between 55 and 60 ma. per tube. Over the years I have tried the Old School Genelex, Mullard Tungsol G. E. Sylvania and Svetlana and Re-issue Genelex, Tungsol and Electro-Harmonix. The tube that I liked the most was the Old School Tungsol 6550 for my 60 watt amps.