That reading sounds funny to me also , the replacement diode is good for 1 amp of forward current max . It will work fine in there , the old one might be feeding AC into the bias section causing the 60 cycle hum and also weakening the vertical oscillator causing the output to be less . I would say change it , you have nothing to loose on it .
let me know how it goes .
Tubejunke , Does that look like your tube list and chassis layout in my previous post ?