Target bias dictated by amp or by tube?

You might try splitting the difference, during your tests(ie: around 90 - 100mA). If you don’t notice an audible reduction in presentation(lost dynamics, rounded off highs, poor focus, etc), compared to the manual’s spec, leave it there. Why waste tube life?

@divertiti , I've been listening to the Cossors on my LM508 for a couple of weeks now, biased at the LM suggested 120ma. Naturally this post grabbed my attention.Have you seen an advertised bias current for the Cossors? The testing data sheets for all 4 tubes say measured - 60ma at the specified plate voltages. Spec or QA test results for matching pairs?

@brotw The voltage should be dictated by the amp as only the amp designer knows what the real plate voltage is. Go 120ma my friend 

@divertiti 

Many thanks, figured as much. Liking these Cossors.