From the owner's manual to the Steelhead;
An effective means of varying the load seen by the MC cartridge has been included in the form of a 5-position rotary switch. This switch selects various taps on a specially manufactured dual-primary bi-filar wound, high-bandwidth low-resistance and multiple-shielded nickel-core step-up autoformer. A drama to make, the autoformer permits the minute MC cartridge signal power to be efficiently and transparently transformed from low-volts/high-current to high-volts/ low-current. By avoiding conventional parasitic cartridge termination resistors, none of the MC cartridge’s tiny signal power is thrown away before amplification. This results in improved system signal-to-noise ratio. Quite worthwhile provided, as in the STEELHEAD, the autoformer has the necessary performance for the job. This pivotal component has had engineering attention lavished upon it in the only way possible or practical: The Manley Labs magnetics department. In-house transformer prototyping and manufacturing capabilities permit realization of extraordinary transformer designs.
Since an autoformer only differs from a transformer by having no secondary winding I don't understand your comment Lewm.
Also from the manual:
5. SWITCH-SELECTABLE AMPLIFIER GAIN
Cartridge output levels and downstream line-level interconnect drive voltage requirements can vary greatly between manufacturers. Hence a four-step amplifier-block gain control has been included to accommodate these differences, as well as differing cartridge sensitivities. You may select from 50 to 65 dB of gain in 5 dB steps. The gain figure is referred to amplifier gain at 1 kHz. Notice that the pre-amp gain is about 20 dB higher (10 times) at 20 Hz and about 20 dB lower (0.1 times) at 20,000 Hz. The MC step-up autoformer may also provide approximately 2 to 12 dB of additional voltage gain depending on cartridge source impedance and load switch setting.