And where exactly did you hear they used Metal Zone on those albums?
From the Opeth's official website:
"Amps: Marshall JCM800 Lead50W, Vintage 4/12 Cabinet, GK Bass Transistor/Line
Effects: Boss Heavy Metal, Turbo Overdrive/Distortion, Volume pedal, Ibanez Pan-delay, Blubber-wah".
Didn't notice...