Amplifier knobs...where to "set" on the clock?

 Speaker Level Controls:
In addition to speaker A/B selectors that permit one-touch selection of two sets of stereo speakers, the M-70 features independent level controls for the left and right speakers in two stereo pairs. That is, you have independent left and right speaker level controls for speaker pair "A", and left and right level controls for speaker pair "B". These controls let you independently set the maximum power level that will be sent to the respective pair of speakers, protecting low-power-capacity speakers from excessive power levels, or matching the output level of two sets of speakers of different efficiency. Independent control over left and right speaker level lets you preset the balance of your system for optimum stereo imaging depending on the positioning of your speakers and the acoustics of your listening room.

Back in the day when the M70 was produced, speakers and equipment were both produced with specs that were questionable at best, and made setting up a system that played well together hit or miss. Being brand specific was the safest bet, and even that didn't always work out. Yamaha came up with a novel approach to part of the problem.