Correlation between tube bias and hiss?

I have a Dynaco ST-70 that has performed well but now has an occasional hiss in one channel that lasts a few seconds. I have switched preamps, cables and speakers and isolated it to the amp. I checked the bias on both channels, which is to be set to about 1.56V. However, neither channel, fully adjusted will go above about 1V. Is this a sign of bad tubes? Might the hiss be related to this?