Very low THD, I don't understand your concern, really:
https://www.soundandvision.com/content/marantz-sr7011-av-receiver-review-test-bench.
Also, there's the audio.com.pl test from above that barely shows the 2nd harmonic around -90dB; for a powerful amplifier this is a very good figure. Feel free to search all amps tested by the guys from audio.com.pl and you'll not find much better graphs (this is also due to their testing equipment).
Sometimes measurements are not telling absolutely everything about the output sound. Feel free to test your hearing with some audio files:
https://www.audiosciencereview.com/...can-we-trust-our-ears.3884/page-4#post-136991. If you can hear the -60dB for the 2nd and 3rd harmonic, then I can create more files for you and see how low can you go.
I'm sure you'll not be able to hear the 2nd harmonic at 60dB away from the fundamental, so why worry about the -90dB from your amp?
You can anytime connect an external DAC to this receiver if you want better THD specs, although...if you'll use a mic & REW you'll probably not notice any differences. Yes, your speakers are having much more distortions than DAC and amplifier.
My ODAC is
the most noisiest DAC from my home and this DAC has several tests and measurements that prove that it's a good and transparent DAC. Well, those ODAC measurements are all good and as long as THD is below 0.1% and background noise with at least 80dB away from the listening level I don't think anyone should car much. Amir is measuring audio stuff so we can choose our best price/performance ratio for our home and budget and to find any possible flaws (safety reasons); 1 or 2 dB between the testes devices doesn't really means something to our ears for sure.
P.S.: There are engineers that are looking to create amps with several harmonics added:
http://www.pmillett.com/3B7.html and same engineers are building amps with very low THD as well:
http://www.pmillett.com/butte.htm.