Strange buzz issue!

Run a good ground wire between the two. Cost 1.00 usd. crimp an eyelet or spade on either end of a good copper # 16. Mount it direct, chassis to chassis.

Regards

Hello,
First,
 Are the amp and preamp on the same power strip. Try a cheep power strip so the outlets are not fully separated.Second, 
 Sometimes changing the tubes helps. On a preamp that does not cost a lot. Next would be the ones on the amp. That will hurt a little. Third, 
Is there something on the circuit causing the hum. A dimmer switch will do it. 

Thanks, I tried both of the above suggestions, but no luck. I today went and bought a hifi grade ground loop isolator (for implementation on the interconnect between pre and power amp). It did the trick! No buzz. But regrettably left me with impaired bass response. Clear sound otherwise, though. Damn ground loops😁🥵

@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.