A bit off-topic, but...
Those Norwegian bands used usually the best amplifiers they could get their hands into at the time. How the sound was then molded is completely another topic, though. Marshalls (JCM800/900) were often used, and many of them used what Pytten (the studio owner) had in Grieghallen- including also that infamous Peavey Bandit. And yes, Metal Zone was very popular in front.
Mixing was another thing. Pytten had the tendency on many albums to make one guitar very thin and evil-sounding while the other one took care of the low-mids. The lows were usually very absent from all guitars. Those combined together, accompanied with the phase-canceling room of his and a lot of cheap hall reverb made the overall guitar sound as we know it. But Pytten had also the tendency to make each album sound different (one of things which made him so awesome) to each other, so your experience may vary. Comparing e.g. Burzum's "Hvis Lyset Tar Oss" to Aeternus' "Beyond the Wandering Moon" is rather pointless, even though many similarities can be indeed found.
For what it comes to reproducing the Grieghallen sound in general, it's also completely doable to an extent. Here are some fun experiments done with Bias Amp Desktop (v1).
The upper one is Hades' "Again Shall Be"- guitar sound switching between the album and emulated versions.
The lower one is a clip from Darkthrone's "Under a Funeral Moon" with also bass included-starting crossfaded in halfway in the bass break and then crossfading back to the real one after the drums creep in.
Only saw your post just now. Thanks a lot for sharing your knowledge on how these sounds where crafted! I know whom to ask when I tinker with Blackmetal guitar tone again .