Strange buzz issue!

@hshifi
Both of these pieces are solid state.

To the OP, please ensure that ground wire between the two is a real good metal to metal contact.  Also you might try shorting out any unused RCA inputs, take that cheap interconnect you have layin' around and cut the end off about an inch back, strip the wires and twist together, plug it in.
It's a long shot, please do this with the units turned off.  In fact make up as many as you need and short all of the RCA inputs (so there are no inputs) and turn them back on.

Regards,
barts

ARC D-100B power amp For fun I connected the LS9 to two other power amps (Yamaha and Vincent). Dead silent. I then connected an old Yamaha pre amp to my D-100B. Silence. Same place, same cables. So, neither the LS9 nor the D-100B are faulty, but together they, or one of them, cause a pretty loud buzz. I’m going mad here! Any ideas? Thanks!

Apples and oranges. Circuit grounding designs of the Yamaha and Vincent unknown.

Looking at the schematic wiring diagrams for the ARC D-100B power amp and the ARC SP9 the B- and signal ground are connected directly to the chassis and thus to the AC mains safety equipment grounding conductor. A poor design. Was a prime cause of ground loop hum.
(AC safety equipment grounding conductor shall be connected directly to the chassis. Circuit ground, B-/signal ground, does not.)

Also I noticed on the preamp two capacitors are installed from each of AC Lines to the Chassis/safety equipment ground. They could be leaking AC voltage to the chassis possibly causing ground loop current through the signal ground via the ICs to the safety equipment grounded power amp.

ARC D100
https://www.arcdb.ws/model/D100
ARC SP9
https://www.hifiengine.com/manual_library/audio-research/sp-9.shtml

Fix?
The easiest thing to do is to lift the equipment grounding conductor at the preamp. Either at the AC plug or inside the preamp where the power cord equipment grounding conductor is connected to the chassis of the preamp.
Check for ground loop hum.

For electrical safety to prevent the possibility from an electrical shock hazard install a minimum #16awg copper wire from the chassis of the equipment grounded power amp to the now non safety equipment grounded preamp chassis. Make sure the connections of the wire to the the chassis are to bare metal for a good 100% ground connection. (The single equipment grounding conductor from the power amp to the preamp ties the two chassis together by the single equipment grounding conductor from the power cord of the power amp.

Check for ground loop hum.

@jea48 is correct. When this amp was built it had an ungrounded power cord. Someone replaced it and now the amp is grounded to the building electrical ground (per EU Directives).


The trick is to have the audio ground of either the amp or preamp be separate from the chassis ground, with a resistance between the two that allows the audio ground to float at chassis potential, yet is a high enough value to prevent significant ground loop currents. 100 Ohms is plenty high enough! But because the amp is using the chassis as ground you'd have to find all the places the circuit is grounded to the chassis, this kind of repair (since this really should have been done when the amp was built) might be tricky. A technician might be able to look at it and let you know.

Plan B: get an isolation transformer to isolate the ground of one of the units. It would be cheaper to do this for the preamp rather than the amp.

If you used a ground isolator, the PS ground should fix the problem. It is the voltage offset between the two PS on the two pieces. What is common without a common ground, the audio circuit ground. You may have ran a wire but it was not to the right place, I'm thinkin..

A ground isolator in the signal path is the fundamentally correct way to eliminate system ground loop problems. ... For an unbalanced interface, the transformer effectively stops any current flow in the cable caused by ground voltage differences, which stops the noise.

I would look to the VAC ground between the two, just by moving alligator clips to different locations.  Might surprise you.
I'm also thinking eliminate the ground on one unit (via cheater plug). Then run the ground wire between the two and only use the ground on the other unit.. I expect that will do it... A little cap trickle.. Common single wire ground should do the trick..

If all else fails a GL eliminator, but it can KILL the dynamics of some systems.. Sometimes the signal won't activate a Sub when there is a GL eliminator in the circuit.. Dayton plate amps.. I know what your talking about, "impaired bass response". I have to use one on a cable box power supply.. It is the actual CC fix.. 

Just an old mechanic thinkin' out loud..

Regards