Audio Innovations Series 800 SUT - looking for info

The switch most likely has nothing to do with "loading".  The switch more likely indicates the recommended internal resistance of the cartridge. In other words, for a cartridge with an internal resistance of ~3 ohms, use the 3 ohm inputs.  For a cartridge wtih an internal resistance of ~15 ohms, use the analogous inputs.  The choice of inputs then determines the turns ratio between the primary and secondary windings, which in turn determines the voltage gain at the outputs.  Since a "3 ohm" cartridge can drive a lower phono input impedance than can a "15 ohm" cartridge, the turns ratio in the former case is likely to be higher (i.e., more voltage gain) than the turns ratio for the 15 ohm cartridge. Somewhere you can probably find what the respective turns ratios actually are.  The numerical values of 3 and 15 ohms are not absolute, meaning you don't have to use cartridges with internal resistances of exactly those two values.  For the 3 ohm inputs, probably anything from 2 to 8 ohms works fine.  For the 15 ohm inputs, probably 10 to 20 ohms.  If you can find out the turns ratios, you can then calculate what works without guessing.  You want the cartridge to see an impedance at least 10X its internal resistance, and the impedance seen by the cartridge will be the resistance at the phono inputs (almost always 47K ohms) divided by the square of the turns ratio.

Ah, the joys of SUT ownership! I have several. I never bothered with the specs. They all sound different, as do the mc cartridges.

I expect the S1000 is wired the same as your S800.  The switch on my S1000 connects a pair of transformer windings either in series or in parallel.  I removed the switch and simplified the wiring for a nice improvement in sound quality.  I later replaced the 1980s RCA jacks with new Audio Notes for another improvement.