Ahhh the nerds at METRO forum for the win!
Roth used a '72 Marshall super tremelo and 4/12 Marshall Bass cabs ( Higher wattage celestions, maybe 65 or 70 watt speakers ?). Roth, believe it or not, didnt use a gain box much, prefering the 'output' tube saturation that his Marshall gave him. This wasnt alot of gain. Roth did have 2 primary Boost boxes though, a sillicon Dallas Arbiter Fuzz Face (used alot on "Fly To The Rainbow' and "In Trance" (End of 'Top of the bill' for ex.) )and first appearing on "Virgin Killer's 'Polar Nights', a Roland Jet Phaser. Roth continued to use the Jet phaser throughout the Scorps and his solo career. Examples are, "All Night Long", Polar Nights, 'We;ll Burn The Sky' 'I got to be free', 'Earthquake', and 'Just another Rainbow'. The jet phaser has a super high gain type distorto/fuzz combined with a great phase tone.
"Sails of Charon' was just loud Marshall, white strat and a wha (sounded like a cry baby as opposed to a vox) Roth used this combination alot. Here again, not as much gain as one would initially think. I do agree with Dave though, a DOD 250 can get you at that type tone as well. To me, the 250 can make a 'clean' type Fendery sound get that great 'cranked Marshall' sound. It particuarly sounds good with single coils.
Not sure if it is fact but I hear all those things. So crank that Marshall and limit the flub and hit it with a treble booster I guess..