Preamp with two volume controls?

You shouldn’t change the subs’ gain once matched.

I used to do it remotely with a MCM 50-8394 before I wised up. You really just need RC EQ so you can change it for each track. Or a simple RC gain control as above.. Or just keep overspending and convincing yourself it’s the best way. I don’t think you will find what you are looking for as it has no reason to exist.

First Sound preamps are quality units with left and right volume controls it that's what you're looking for.

https://www.firstsoundaudio.com/

Older Don Sachs have dual volume control 

That was it.

Audible Illusions.

Audible Illusions is based in Daytona, FL. and their tube preamps are well regarded. Also, their preamp has two "balance control" knobs (one for each channel), in addition to the master volume control.

The owners manual describes the balance controls as follows: "The amount of signal voltage applied to the line stage for each channel is regulated by separate stepped attenuator controls. The relative signal of the CHANNEL BALANCE is set with these controls. Setting both of these controls at 12 o'clock is a good starting point."