I guess then, when considering an amplifiers rating into an 8ohm load, one should half that rating into a 16ohm load as a way of assessing that amplifiers ability to provide enough power for a particular circumstance.I always found that the ST-70 played better bass if it was possible to use the 16 ohm tap. This is assuming of course that the speaker was at least 16 ohms in the bass region!
The power rating of the amp actually goes up a watt or two as the output transformer is more efficient when used on a 16 ohm load.