The Yamaha R9 seems to be fully functioning, just missing it's door. I'm not sure why there's two volume controls.
My concept 7.5d still technically functions fully. But it causes woofer pumping. Issue is gone if I use another preamp fed into the power amp section.
Your R-9 has a (flat) volume control and, concentrically mounted, Yamaha's long-time signature
variable loudness control. The latter approximates the Fletcher-Munson (etc.) curves, but allows flexibility and adjustability that most "loudness" controls of the era didn't. The idea was (is) this: with the loudness set "flat", turn up the volume to the loudest level you're likely to routinely listen to. Thereafter, use the
loudness control to reduce the sound output level (rather than the volume control), thus -- theoretically
-- giving you reasonably 'accurately perceived' frequency response irrespective of SPL.
If nothing else, it's more flexible than most loudness control schemes.
The concentric volume/loudness is nice. Typically, Yamahas had a
big volume control and a smaller, separate loudness control.
See, e.g., my much beloved CA-610II integrated amp, which I have owned since it was new (1978).
P1020541 by
Mark Hardy, on Flickr
As an aside: Variable loudness was
not uncommon in the early days of consumer hifi ('50s into the early '60s).
EDIT: If you don't have a manual for your R-9, you can get one at
https://www.hifiengine.com/manual_library/yamaha/r-9.shtml
Free registration required to access their content, and well worth it IMO. If you have any trouble registering, send me a PM and I can offer some form of assistance
Also -- your poor Concept. Sounds like LF oscillation motorboating) or maybe DC getting through to the speakers
Not a good thing -- but nice to know you were able to isolate it to the preamp section.