Effect of under-bias on tubes and sound

Literature indicates that under-biasing of output tubes can cause unpleasant crossover distortion. However, how does one determine what is too little bias? I am running an Audio Research Classic 30 which utilises 6550 output tubes. Standard bias is given as 32.5 mV. I'm able to set the adjustment pot to it's maximum to get a reading of 27 mV which is 5.5 mV below spec. (17% deviation). Any idea whether this falls within acceptable parameters? Or is my option to take it for measurement of distortion, and possible component (tube) replacement? Sound seems clear and and dynamic but degradation can happen gradually, and one's ears tend to accommodate the changes to some extent.