Preamp with separate volume control for each channel?

Um, it would seem like your amp was not repaired? It should be working properly. I don’t understand why people are suggesting solutions to remedy a problem that any half decent tech should be able to fix? 
 

Send it back, and have them fix it properly.

@perkri 

+1

I have a Vacuum Tube Audio, aka tubes4hifi, SP 14 line stage. There are options when you order one, but mine has remote control volume, a master volume control, and separate trim pots for left and right channels. I like the sound a lot. It also has two outlets so biamping is easy. Highly recommended. I ordered mine with os Sylvania tubes.

Fix it right.  The RM9 is worth the effort.

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.

I tried one of these units when they were new.  While very transparent and resolving, there was a great deal of channel cross talk if you had more than one source connected.  Plus there is a very cheap feel to these.  Could be a decent solution if you could set and forget it in a processor or tape loop of an integrated amp instead of using as a true preamp with multiple sources connected.