HDAM's
The point of the following three paragraphs is that these circuits exist, not to understand their operation:
The HDAM's used by Marantz in AVR's and AVP's are diamond buffer four transistor circuits with (4) resistor current sources. In AVR's the supply voltages are +/- 7V. In the AVP's the supply voltages are +/- 12V.
One output buffer circuit for that supports one RCA output is made up of an HDAM, complementary voltage gain stage (two transistors) and then another HDAM. Feedback is taken from the output of the 2nd HDAM and input to the low impedance node of the first HDAM.
The (-) output leg of the XLR outputs of the AV8805 and AV7705 is made up of (3) HDAM circuits plus the complementary voltage gain stage.
Thoughts
Diamond buffers are well known circuits.
The current sources (or current mirrors) should be implemented with active devices such as transistors. Resistors make mediocre current sources, especially as the circuits become more heavily loaded. Voltage sources attempt to supply whatever current is required to produce a desired voltage. Current sources attempt to supply what ever voltage is required to produce a desired current.
The supply voltages should be +/- 12V minimum.
IC Opamp Alternative
Opamps such as the venerable NE5532 dual opamp would likely provide better performance with +/- 12V supplies. One of these dual opamps would replace two HDAM buffers (4 HDAM's and associated components) and save significant board space as well.
One NE5532 (two opamps) would be required to support the XLR outputs of the AV8805 and AV7705. The NE5532 would replace (5) HDAM's and associated circuitry.
From one distributor chosen at random, NE5532's cost 15 cents in quantity 2,500.
Better opamps are available, but other components (DAC IC's, opamps in the DAC filter circuits, layout) in the AVR's would likely have to be improved before the better opamps would make a significant overall improvement.