If the hiss follows the tube, than the tube is likely the issue. If I am understanding correctly, this is not happening. One tube/socket combination yields hiss, switching the tubes giving the alternative tube/socket combination does not. Carefully check the pins on the "hissing" tube for cleanliness, straightness, etc. A different pair of tubes would be helpful to identify or eliminate a tube as the issue.
Do you have a DMM that measures frequency? If so measure the frequency of the hiss at the outputs. Hiss is much higher frequency than hum. 60Hz hum is grounding issues, 120Hz hum is transformer. Much higher frequency hiss is most likely the tube.