As far as clean anything is possible. Zakk is known for rolling the volume down and stepping off his sd1 but I bet there was another amp in the studio dubbed. Please I’m not a Randy staub lol so take this as a grain of salt but I’m willing to bet the Spx 90 is a mono in stereo out unit and I believe the exciter is also. Could be wrong about that but either way it was probably sent using an aux or using the busses on an ssl console returning on two stereo channels.
From there the effects “ Spx in symphonic preset and the exciter are Eq/compressed on their corresponding return channels then sent to other effects such as lexicon 480 or 240 or plate reverb any h3000 etc whatever was available at the time to help blend and gel yet give it that thing. Then maybe even widened with the ssl stereo channel feature and tucked back in. Then using automation was blended in either at a static setting or during some pinch squeels during key parts ridden louder to make those little licks, leads, parts by themselves stand out and push out of the speakers. They weren’t used in the amps effects loop. Again please I’m not bob rock but this is my best guess lol how I would do it.
And I think zakk doesn’t use chorus during tracking he doubles/ triples his parts. The chorus sound is the Spx in the studio in the old days.
hope this helps
Again my best guess.