It seemed to me that, as fas as power is concerned, the A-S301 would be the most fair contender. If we compare the Onkyo A-9150 to the A-S501, then the power difference is even bigger, but for the price it seems a very nice amp.As far as power is concerned, but most folks compare what they can afford under a specific budget. Dollar for dollar, the Yamaha has more grunt.
Comparing the 9150 to the 301 is akin to comparing a Toyota Camry to a Honda Civic.
I'm still curious as to how Onkyo can build a 4 ohm stable amp for $500 and it takes Yamaha $2500, their lesser priced amps using a speaker impedance switch. I can't remember when this switch first appeared on an amp, but I don't believe it was on what was considered to be a quality amp.
The Onkyo just has a built-in power limiter. These switches/devices are not reflective of an amp quality. They exist solely to sacrifice the speakers instead of the amp in case some idiot does something like attempting to drive 2 ohm loads to 120db. They are simply idiot-proofing. It allows the manufacturer to stamp a UL or similar rating on the amp. If these switches are left in 8 ohms they can still drive 4 ohm loads without detriment, it only makes it more likely that the amp will also suffer damage if overdriven. Speaking from personal experience, the $500 Yamaha integrateds can drive <4 ohm loads to high SPLs without breaking a sweat. As for amps that double on power as impedance halves, point to any new-production product that does that for <$500.