My first findings
Separate center reference voltageThe A6 seems to use a reference voltage, PMID/PDMID in place of ground whenever ground would be expected on any of the inputs of an opamp. My guess is that PMID/PDMID means Power Middle/Power Digital Middle or something like that, as ground in these places would normally be the center of the wave.
The PxMID originates with a 5V voltage regulator. I've calculated PxMID to be 2.05V. The A6 uses +/- 12V internally, with an offset of +2V this means that any positive amplitude is reduced to +10V minus any opamp limitations.
The PMID line is littered with bypass caps. I assume that by using a separate, well regulated reference voltage, one may get rid of some noise from other components, and also that its hard to get 0V from a regulator, thats why it's 2.05V (?).
The noise source seems fairly standard, it is based around a zener diode (which is the same technique as using a transistor without connecting the collector). The pink noise filter seems to be a true pink noise approximation (3dB/oct) as it uses a multiple filter sections in the same fashion as Elliot Sound Products. It is not entirely similar as it has the filter sections connected to between the positive input and ground, whereas the Elliot version have them in the feedback loop, but I assume this is only because the A6 one is a non-inverting filter and the Elliot an inverting filter. All noise outputs are connected to an inverting amp which also adds the PMID reference and acts as a low pass filter with cutoff at 493Hz.