All I can tell you is that I had a Yamaha CR-1020 that I bought new in 1977. It was second from the top of their receiver line and Yamaha rated at 75 wpc. I loved that beast! I was even offered more than I paid for it about ten years ago, being told by a couple of technicians that the build quality of that amp, as far as a solid state receiver, was second to none. Unfortunately, I lost it in a fire 5 years ago.
The 275 watts is the unit's power consumption, not output. I had a '75 CA-800 integrated amp until 2 years ago, it was rated at 45 wpc in Class AB and 8 watts in Class A. It met distortion specs at 60 wpc. If your A-500 (CA-500?) is from the same series, I'd bet on a 30 wpc rating.
I have the yamaha AVX500 integrated amp. It sounded great with my KG4s. Then I got the upgrade bug and bought RF3s and a yamaha 5240 receiver. Talk about harsh! I'm selling the yamaha and getting the Denon
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