The Sunn reverb recovery circuit uses a single transistor stage for gain and the reverb control is in the collector circuit - with DC flowing through the control. The DC through the control can cause some noise, even with a clean control when it is rotated, and when it's dirty the noise can become excessive. If the transistor is also noisy itself, it can introduce a lot of hiss even with the reverb control turned down assuming either a dirty control pot, cold solder joint, or something that puts some amount of extra resistance in the collector circuit.
You might want to clean/replace the reverb control and/or replace the recovery transistor.
