If I am reading the Onkyo 805 user manual correctly, the settings you referred to, Keith, are for configuring the amplifier to properly drive speakers whose impedance throughout the audio band falls within the 4 ohms to 16 ohms range or the 6 ohms to 16 ohms range. The impedance plot of the ST's shows that their impedance falls within the 4 ohms to 16 ohms range for the entire audio band, with the exception of rising to 17 ohms from approximately 1.4 kHz to 1.6 kHz. The worst phase angle within that frequency range is approximately -23 degrees. 17 ohms at -23 degrees is a relatively benign load on the amplifier, so it shouldn't make any difference which setting you use with that load. However, given that the ST's impedance drops to 4 ohms in a couple of sections of the audio band, the better setting for your amplifier is the 4 ohms to 16 ohms range. There you have my two cents worth.
Regards,
Bob