Which amps offer the best distortion linearity?

What I mean by distortion linearity (possibly the wrong term) is the most consistent (and lowest) distortion vs power across all frequencies.

Here is the graph from the NCx500 review:

The maximum SINAD before clipping (@15Khz) is about -75dB.

Here is the same graph from the Nord Three SE 1ET400A (Purifi) review:

Again, maximum SINAD is about -75 to -80dB for 15kHz.

Now here is the graph from the review for the Soncoz SGP1 (class AB):

Apart from the 20Hz curve, which was apparently fixed, the performance seems very well behaved (about -85dB max SINAD @15Khz).

One that fares a bit worse is the MiniDSP SHD Power (just for example):

A max SINAD of -55dB @15Khz.

...and of course AVRs also tend to also score quite poorly in this, for example the Denon AVR-X8500H:

Although it behaves predictably SINAD is up to -65dB @15Khz.

So if I'm picking a power amp to improve on this aspect of the AVRs internal amp performance, which models would be best?

The other requirement is to preferably exceed the AVRs 2 channels driven power of 225W (into 4 Ohms).

Edit: Is it fair to compare amplifiers by these graphs? What is the effect of different amplifier gain? Some amps are probably measured in low gain mode which wouldn't be suitable for use with an AVR.

What I mean by distortion linearity (possibly the wrong term) is the most consistent (and lowest) distortion vs power across all frequencies.

Here is the graph from the NCx500 review:

The maximum SINAD before clipping (@15Khz) is about -75dB.

Here is the same graph from the Nord Three SE 1ET400A (Purifi) review:

Again, maximum SINAD is about -75 to -80dB for 15kHz.

Now here is the graph from the review for the Soncoz SGP1 (class AB):

Apart from the 20Hz curve, which was apparently fixed, the performance seems very well behaved (about -85dB max SINAD @15Khz).

One that fares a bit worse is the MiniDSP SHD Power (just for example):

A max SINAD of -55dB @15Khz.

...and of course AVRs also tend to also score quite poorly in this, for example the Denon AVR-X8500H:

Although it behaves predictably SINAD is up to -65dB @15Khz.

So if I'm picking a power amp to improve on this aspect of the AVRs internal amp performance, which models would be best?

The other requirement is to preferably exceed the AVRs 2 channels driven power of 225W (into 4 Ohms).

Edit: Is it fair to compare amplifiers by these graphs? What is the effect of different amplifier gain? Some amps are probably measured in low gain mode which wouldn't be suitable for use with an AVR.

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