Here's' the FFT results of the 10 kHZ Uldo-Nacho and the Focusrite 2i2:
The sample rate is 96 kHz, 24 bit. FFT is a 1M pt 10 sample average.
The output is 0 dBu loaded only by the notch filter and A/D.
The red trace is the notch filter output. The measured values do not reflect the +40 dB gain of the post-notch amp.
(The 30 kHz spur is actually -125 dBc/dBu.)
The hash in the notch filter output is due to long capacitor leads, wrapped construction of the polystyrene caps and lack of sufficient shielding.
I paid no attention to "outer-foil-to-lowest-impedance" on the caps and the board is only shielded by a sandwich of cardboard-clad grounded aluminum foil.
The calculated H2...H4 HD of ULDO-Nacho is -119 dBc relative to 0 dBu or just over 1 ppm.
With the oscillator running at higher level and using a pad to obtain 0 dBu I expect it to measure slightly lower.
The blue trace shows the Focusrite 2i2 (Gen 3) A/D HD at 0.0002% or about -113 dBc.
Note that the H2 is introduced by the A/D.
Overall ULDO-Nacho is about 6 dB "better" than the A/D which says a lot for both of them particularly the 2i2.
I've got some 1 nF 1% stacked polypropylene Wima's in my Mouser cart for my next 10 kHz build.
I'm also going to try 100 Hz.