One minor change, each 1M ohm resistor in the HDAM circuits has been changed to 100k ohm. The effect is to lower the impedance of the circuit that includes that resistor and a low value capacitor at lower frequencies since the two are in parallel.
The impedance of the two component circuit, which is connected from the signal circuit to ground, will now change less from lower to higher frequencies. From 20Hz to 20kHz the impedance change was 27.9X, now the change is 3.8X. That circuit likely provides compensation for the overall buffer circuit.
Thank you for the information, you are amazingly resourceful! The service manual was not available for purchase last time I check.
The thing that bothers me is simple logic. The HDAM is obviously a buffer that is not meant to double duty as a sonic equalizer. If it is, then it would be a fixed one, not adjustable.
So it is unlikely to be responsible for injecting 2nd harmonics or something like that to project a "warm" sound. FR, FFT etc., did not show any sign either though one may argue all of the FFTs we have seen so far were from a 1 kHz stimulus test signal so those may not have revealed the magical harmonics.
Also, the chance for such discrete buffer stage to have SINAD better than the upstream DAC and/or Volume control ICs would be 0 or virtually 0. Even if it has SINAD of 120 dB, the best it can do it not degrade the signal at the input to the HDAM at all, but cannot improve it. Back to logic, then if the HDAM is so important regardless for the sonic signature according to Marantz, why are they not used in the Slimline and their two channel receivers? So I won't say Marantz marketing lied about the effects of those HDAMs on the unique warm/musical sound signature of their AVRs, but I will say those who read the material or told about it via magazine review and/or internet hearsay and then heard the advertised/reported sound signature likely heard it because of the influence/expectation bias.
I do remember reading a block diagram, or schematic of one of their PM integrated amp, forgot the model#, and noticed that the HDAMs were upstream of the DAC and Vol control so my argument/logic obviously won't apply in those cases, unless I remember wrong.