That's more likely to affect low-frequency hum or buzz (dealing with 50/60Hz mains interference and/or harmonics thereof), rather than hiss.
Hiss usually points to higher-frequency noise, more often originating in leakage-causing/conducting subtances in the high-impedance areas of the circuit. Which is not unexpected (perhaps even likely) in this dog's breakfast of a physical design
Easiest thing first - try some other tube(s). If the noise stays the same, that points to the thing(s) that stay the same, ie. the circuit and/or construction.
