Throw away your APx555s! Just kidding. Actually, there is some truth to this, particularly if you interest is SINAD and audibility from a scientific perspective. Considering that most of our listening with with CD-quality (or worse), which has a limit to 16-bits. 16-bits is 96db of dynamic range. So really, we don't need measurement gear that does better than that. Even moreso when we consider that the signal to noise ratio of microphoned instruments or electronic instruments plugged into mixers is probably no better than 55-85db. Which makes us wonder if we should not just cut off the lower part of the graph below the -100db line. Think of it like tolerances for automobile body panels. What's considered acceptable tolerance for the gap? 3mm, 1mm, 0.5mm? I'm sure Toyota's spec is probably much tighter than Ford's or Chrysler's (Tesla probably doesn't even have one, their spec manual likely being "use eyeball" or "whatever Elon says is OK for the day"). However the fact is, after a certain tolerance spec, it's not going to matter to the human eye. I'm not going to care if the gap between the hood and front quarter panel is 0.01mm off from spec. The reason is because I cannot see this. This is why taking measurements where we care about -120db spuraie is utterly ridiculous. While I do think maybe there is some information that can be gleaned indirect from such a spectrum analysis as a whole; anyone who tells you that the aggregate of the harmonics and "grass" below -100db (or -80db even), aka SINAD, can be used as a measure of audio quality, is either lying to your, has an ulterior motive, or has illusions of grandeur. Most likely the latter. There were some of these people in the past: Doug Self, NWAVGUY, etc., and there will be some in the present, and in the future. The fact is, modern A-D chips do a damn fine job. A good enough job - to take simple SINAD type measurements. Case in point, the MOTU Ultralink mk5. Let's start with the APx555 taking a steady-state sine measurement of the Schiit Modius ES DAC. I will include measurements from the APx555 with the the High Performance Sine Analyzer turned on and off. The HPSA on the high-end APs is kind of an ingenious cheat that gets us better results when measuring a single sine signal. The HPSA comes in very handy if one really wants to mastubate over SINAD type readings. Schiit Modius ES DAC 1kHz steady state tone at 0dbFS (red) Y-axis not normalized to anything in particular 32k FFT, 10 average APx555 (High Precision Sine Analyser On) Schiit Modius ES DAC 1kHz steady state tone at 0dbFS (red) Y-axis not normalized to anything in particular 32k FFT, 10 average APx555 (High Precision Sine Analyser OFF) Now here's the MOTU Ultralink mk6, into the front panel XLRs with the pad turned off (it must be for hot XLR line signals). Note that the APx555 is $35k these days. The MOTU is $600-$700. Schiit Modius ES DAC 1kHz steady state tone at 0dbFS (green) Y-axis not normalized to anything in particular 32k FFT, 10 average MOTU Ultralink mk6 with ARTA software The APx555 has less random grass / spurs. However I'm getting a lower noise floor (and lower 3rd harmonic) from the MOTU. Surely this cannot be considering both are using a 32k FFT size? Actually, it's probably because I'm using a different algorithm Kaiser 7 for calculating the FFT in ARTA. I have the AP set to use their secret proprietary algorithm (which is super fast). Let me use a higher FFT size to get the same "noise floor": a 96k window size. Yes, setting different parameters with how the FFT is calculated will give us different results! This is one thing which is very important to note! (And also one way to give reader misleading results if they are sloppy or don't fully document the parameters of how they arrived at the measurements). Schiit Modius ES DAC 1kHz steady state tone at 0dbFS (red) Y-axis not normalized to anything in particular 96k FFT, 10 average APx555 (High Precision Sine Analyser OFF) So here we go, the APx555 using the AP algorithm with a 96k FFT window size gets us about the same noise floor as the ARTA with the 32 FFT size using Kaiser 7. And not just that, the results between APx555 (in its non-cheat mode) and the MOTU Ultralink Mk6's front panel inputs with the ARTA software are very similar to each other, with neither one truly besting the other.