I actually posses a full schematic (with part numbers) of the entire SR570 amplifier. Its not digital so I'd have to find a scanner to show you guys...
Anyway, when running on battery power:
the batteries are fed directly to an LM2940 (10V LDO regulator) for the +10V reference and to an LM340 for the 5V reference (through some MOSFETs that are used as switches). There is an interesting circuit involving an opamp that uses the -12V battery terminal and the regulated +10V to create a -10V reference rail.
I looked up the voltage regulators and their datasheets specify their output noise voltage, which is way higher than the noise specs of the overall amplifier. So the magic must be in the amplifiers they use. The input stage uses two low-noise amps depending on the mode...AD743 (low noise spec), and the AD546, for low input-current mode.
I'm pretty sure all the important bits of the amp are inside a can on the circuit board.
Anyway, I'm not sure whats causing this excess noise. The batteries are well filtered and the regulators are higher noise than them anyway, so they would have to be very very degraded and extremely noisy to permeate into the low noise amps (which, being low noise, were probably designed for extremely good immunity from power supply variations.
I thought it was the batteries but it could be anything, since the gain on the amp is set to 1,000,000 or 10,000,000. anything that couples in just before that point...
the batteries need to be replaced anyway. if it fixes the problem, great, but if it doesnt, I'm still stuck where I am.