Preamp with separate volume control for each channel?

"had my vintage Music Reference RM-9 repaired"

Well sort of... I would take it to a different shop for repair. There are a number of preamps with dual volume controls, but I don't think that is the solution you want.

If it was working perfectly, I would still carry on about the benefits of REMOTE BALANCE, once you have it, you never want to live without it.

Chase Remote Line Controllers RLC-1 can be added in-line or thru a loop like tape or processor or via an alternate input.

You get 1db steps of remote volume, balance, mute, treble, bass, as well as 4 selectable line in; two identical line out (front/rear identical from early 4 speaker with fader era).

You must have the remote, no controls on the unit, none with remote are listed now.

1st, buy the only remote listed, they all use the same one

Chase Remote KP-1, $30.

2nd, buy a unit that has no remote, but 1st have seller confirm all it’s lights blink red when plugged in (not guarranteed, but more than likely it’s ok). Zero noise, I just added a toggle power switch to this one to re-set it to it’s defaults.

You also get automatic and progressive bass boost as you enter very low listening levels which you may or may not like, IF you use it properly.

this old listing has good info and photos

https://www.usaudiomart.com/details/649391586-chase-technologies-rlc1-remote-line-controller/images/1694269/

It sounds like your amp needs to be brought back to proper working order, whether by the shop that should have, or by another.

Khozmo makes beautiful products. Sadly their stepped attenuators won't fit in my space-constrained preamp. Neither will Goldpoint or DACT for that matter. Back to pots I guess 🙂

I have a Spatial Audio Lab Raven Preamplifier. Using the remote allowes for +/- on either chanell. Oh and it’s a beautiful PRE. Good luck.

I have no clue if this idea works for you, but check out the Emotiva PT1 or PT2. Either one will give you a decent DAC, preamp, and pretty darned good balance control that can be set via the remote control for pretty accurate settings of channel balance. Neither are super cheap but for what they bring to the table are quite reasonable in cost and in performance. I love mine and If I ever need a second one, I won't hesitate to go that way.