What I suspect is occurring - is that most amps operate in class a/b which means that its initial gain is in class a but it switches to class b at a very low levels. This switching from class a to b gives your amp the ability to operate up to a high power output but the switching can cause sonic issues some folks are sensitive to. So enter class A where the amp operates only in class A up to its rated power. No switching distortion, lower power, runs hot(er), and costs a lot of loot. A lot!
What I ASSUME Yamaha has done (remember there is no free lunch - no one is giving pure Class A amps for nothing) is that in 'auto class a' the Yamaha's class a is running up to a higher level before you switch to class b, so if you have efficient speakers you will never encounter the problems switching might intoduce. Its still a Class A/B amp, just smoother at higher power levels.
This is very common in Class A/B amps except most don't usually have an option and the class a power levels vary. Some manufacturers have the level high enuf that they try to pass off the amp by implication as pure Class A.
Hope that helps a bit.
What I ASSUME Yamaha has done (remember there is no free lunch - no one is giving pure Class A amps for nothing) is that in 'auto class a' the Yamaha's class a is running up to a higher level before you switch to class b, so if you have efficient speakers you will never encounter the problems switching might intoduce. Its still a Class A/B amp, just smoother at higher power levels.
This is very common in Class A/B amps except most don't usually have an option and the class a power levels vary. Some manufacturers have the level high enuf that they try to pass off the amp by implication as pure Class A.
Hope that helps a bit.