Hey, I am not sure how physically close to an ARC dealer you might be. Try borrowing a Ref 6 or even better the SE version and hear if it happens with the demo. I have the 5SE with the Luxman M10X. I bought them in that order. Maybe your 6 does not match the H20. But the 6se might. The impedance or the voltage is different. I will tell you I not my friend who has a similar setup (both SE) have had this issue. I say borrow the demo from the dealer and email ARC at the same time.
Just an idea to help.
ARC Ref 6 Tube Microphonics
Hi All, I purchased a used Ref 6 preamp a few months ago, initially everything was fine, tube hours indicator said ~950 hours but not sure if that was accurate. After about 50 hours of use, turning the volume past 0 and the mechanical relay kicks in, a chime is heard coming through the speakers. I assumed one or more tubes had become microphonic. I replaced all tubes with new ARC select from Upscale, 7 Sovtek 6H30Pi, 1 Sovtek 6550WE. I let it warm up for a few hours, turned volume past 0 and the same chime is still there. On the positive side, the SQ improvement was immediately noticeable and outstanding, I love the preamp in my setup but the microphonic chime is annoying. ARC service said that some chime is normal but it still bothers me. Any advice? Ignore it? A new set of tubes direct from ARC? New tube rings? Send in to service? Thanks.