Target bias dictated by amp or by tube?

The sound quality will likely be inferior at the lower bias. Bias plays a huge role in an amp's sonic character. Try both but I bet you'll prefer the higher setting. 

If one were to use the lower of the two, which means it would be running out of spec according to the amp, does the amp simply produce less power or will the sonic qualities suffer as well?

The power will be unaffected.

Thanks guys, sounds like the amp spec is the one to conform to.

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?