I don't think there's any question that current amplifier rating standards won't tell you much about how various amps will sound with all channels driven to full power with all channels driven. But how often does that happen in real world conditions? I don't think there's any empirical evidence that typical program material will present that kind of load to all channels at full bast into all channels. I worked for many years at the Federal Trade Commission to come up with a sensible standard for rating multichannel amplifiers, and I finally gave up. The all-channels-driven standard is clearly too conservative. One-channel-driven is cleary bogus. There's a wide range of choices in between, but tax dollars would probably be better spent helping the poor than fine tuning a "correct" standard. That said, I share your frustration with relying purely on blind ABX tests to judge amplifiers. It's extremely difficult to conduct those tests correctly, and the chances of a false negative are quite high. Based on my personal experience, I don't think there is much difference between amplifiers if they aren't taxed beyond their design limits. But we're back to personal opinion, which means we're back to what I suspect will be a never-ending debate.
I haven't been following this thread closely but I find this post interesting since I used to design amplifiers as a EE and now I measure them when reviewing them on this site.
I wrote several articles on the pitfalls of the ACD Test such as this one:
The All Channels Driven (ACD) Amplifier Test — Reviews and News from Audioholics
I also came up with a basic amplifier measurement standard that we use on all products reviewed at audioholics:
Basic Amplifier Measurement Techniques — Reviews and News from Audioholics
As you can see in the above article, we test:
- continuous power into 2 channels fullbandwidth into 8/4 ohm loads
- 1kHz ACD per what most of the print magazines do into 8 ohm loads for receivers and 4 ohm loads for power amps
- CEA 2006 Dynamic Power testing into 8 / 4 ohm loads
I think this combination of testing gives a very good indication of amplifier power capability with the CEA testing being more representative of real world testing. I ran this by the guys at UL and Audio Precision and they really liked it. But there is more to an amplifiers sonic character than just raw power. This is why we measure channel to channel crosstalk, SNR, distortion, etc.
Now as far as amplifiers all sounding the same until they exceed their linear operating region, I firmly disagree. An amp with a high noise floor masks low level detail. I've heard this effect directly when comparing pro amps to high quality home theater amplifiers. I also did extensive testing with Class D amps vs linear amps and how they sound powering bass drivers. To my surprise I found the most dynamic and snappy bass driving the subs in my RBH T30 speaker system using an Axiom A-1400-8 Class D amp over my Denon POA-A1HDCI linear amp. The differences were not subtle and were easily heard by myself and other listeners during ABX testing. That being said, almost everyone preferred the linear amp for the mid/high frequencies. I believe the Class D amp had more ability to deliver instantaneous current than the linear amp which was seen in the dynamic power testing. Oddly however the audible differences still existed even at low power levels where neither amp was even close to clipping.
When it comes to loudspeakers and amplifiers, measurements are VERY important but they don't ultimately replace a controlled listening test to determine what works best for the particular application.
As far as equipment synergy, it does exist! If you have an amp with a high zero gain crossing driving a highly reactive load, it will alter steadystate frequency response and ultimately tonally shift the experience. Many Class D amps lack adequate post filter feedback and are very load dependent.
The bottom line here is don't generalize and think everything is known about a products performance from just a few steady state measurements. With passive devices such as cables that may be true. But, amplifiers and especially loudspeakers and how they play into a room and ultimately integrate back into our hearing mechanism is a more complex process than we can always measure with a high degree of confidence.
