Acoustic Research AR9LS owners? Just for thought

FWIW, I believe when the AR's came out 'depth of imaging' was something hi-fi enthusiasts didn't have in their sights yet. That came a few years later with Harry Pearsons focus of 'sound staging' including things like depth of image and air, etc. Designing the speakers system's bass to sound best when the speaker wall was backed up to the wall was no more that designing it to work as most folks were positioning their speakers then. Consider Allisons, Snells, etc. So it follows that if you pull these speakers out into the room to a locus where the speaker is about 25% or more of the room's length from the wall behind it you may loose a lot of bass, but you might, just might, greatly improve the speakers sound stage, i.e. width, height, and depth. And in practice you could add a sub woofer to compensate. Could. Maybe. :-) FWIW.

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.

Yep. Especially if you can't hear a difference. That would be the only benefit and you might not be the only person who can't hear it. It is rather subtle.

This is just a guess but from what I can tell from your posts it appears that you have the speakers centered on the long wall firing across the short dimension of the room.

I have done that and the result was aways a loss of upper bass -lower mid range energy. While the speaker measured well in the bass the overall sound was cold/bass shy. What I think occurs is that when the speakers are on the short wall there are a lot more reflections/reinforcements and the bass frequencies sound fuller.

Try moving the speakers to the short wall and see what happens.