This is a nifty little tool that's helpful for EQing speakers and finding comb filtering issues.
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I use AudioTools for iOS and the FFT spectrum analyzer. Play pink noise through the system and try to get it relatively flat with EQ. You can also go full hog and make an automatically generated custom profile with REW (Room EQ Wizard).
Even when I invested into display calibration sensor, it was because I am a photographer and having calibrated display is definitely helping when I am doing color grading (its also awesome when watching movies). This is bit different because one cannot become an audiophile while using custom desktop computer with 14 fans, regardless of how silent they can run.
So after reading dozens of specs, SNRs, THDs and such stuff, i will probably not change anything on the current setup. A subwoofer might add some kick, but so far, the burn-in of the speakers (over two weeks) helped the bass, and I placed them in a way where the table helps to pronounce the bass sound.
It is still bit of a surprise that Analog input has slightly better sound than Toslink, but after some further read its not so uncommon - the internal DAC should be flat. Also, according to the NAD support, the analog sources are going directly to amplifier, bypassing internal DSP/DAC. Does it opens a way for an upgrade? Probably not because my current soundcard and AUX input both have SNR at 93db, and about same THD.
I would probably like to see more reviews of NAD d3020 v2, but thats about it.