you are absolutely right, that this DC test is not very realistic for a normal use, but the insight from it is very valuable. I’m even pretty sure that I have opened a little box of pandora here
The measurement is damn simple. I apply a DC offset in digital generator of my AP SYS-2722 and the DAC should present a DC voltage at the analog output (No amp and speakers connected ). If this doesn’t happen and the voltage stays at ~0V, the DAC has a highpass filter in its digital or analog sections.
In order to protect gear like speakers against very slow signals (down to DC) the so called corner frequency of such filter needs to be lets say in the 0.1…1Hz area and this can produce significant phase shifts up to 100 Hz or so, which could be audible (This is the reason, why such a filter is almost always switchable in amplifiers). One could document that behaviour of the Rossini by a more realistic frequency and phase response measurement (I will probably do), but when the DAC is not able to provide DC in the mentioned synthetic test, we have a clear proof, that a subsonic filter is active.
If this is done in the digital domain (FPGA, DSP), a so called IIR filter (in contrast to the FIR interpolation filters used for oversampling) has to be used, which is widely known for its pathological behavior regarding numerical errors and instabilities in its calculations, which would necessitate additional measures and compromises regarding SQ, like dither etc.
With all these problems related to a digital subsonic filter, the question is, why this is not better solved in the amplfier. All good amplifiers have DC protection and some of them have switchable subsonic filters.
Since no other top DAC brand I know, does that and I never heard, that a DC or subsonic signal from a DAC of another brand ever killed a woofer or triggered the DC protection of an amplifier, the question is, why is it in, why can’t you switch it off and why is this feature not marketed ?
This goes to dCS’ engineering and product management.