You have 3 roads here:
- Analog balance potentiometer. Either built-in to a new preamp/integrated, or as a separate unit. This will often be implemented by an Alps Blue-Velvet (RK27) type pot. These are OK in an absolute sense, but when I look at a $4K+ preamp that uses one if these things (they are VERY common) I think "bleh" and move on. You can hear their effect (compared to better pots), and I don’t like how they sound. I have a Rogue Hera preamp with a high quality stepped-attenuator main volume control, and an "opt-in" Alps Blue balance control. I always leave it switched OFF (bypassed) because bleh - Alps Blue, ugh! Did I mention how much I hate those RK27 pots? I have no problem with other pots (Alps RK40, RK50, Nobel TKD), but I don’t know if there are any such good options for balance control. Also: I hate the RK27 Alps Blue pot.
- Digital volume control. Many DACs and software programs have this feature built in. Some digital processors, like Audiophilleo 1, had this as a nice option. Even some high quality analog tube preamps use a digital IC volume chip with this option. ARC Reference and (IIRC) Rogue RP-series preamps are 2 great example here. When I had the ARC Reference 6, its balance control was truly excellent. I also had an RP-9, and though I can’t quite remember (for some reason - put down the booze past me), I think it has a balance control that was also quite good. Meridian preamp/DACs (808i series) and processors (818 series) also have very good digital attenuation with very good L/R control. Most 808 and 818 also have high quality analog inputs (it’s converted to digital, but at very high quality) if you wanted to route everything through them. It might be sacrilegious to convert a vinyl source to digital (and then back to analog again), but when I tried this with an 808v5, it was surprisingly close to transparent (not 100% though).
- Insert a 2-knob dual-mono stepped attenuator control unit (i.e. you can control L and R attenuation independently). You can either use this as your "main" preamp or in addition to your actual preamp. A high quality S.A. is transparent enough that it’s OK to use in addition to a traditional preamp. You will have to run an extra set of ICs. And only 24 steps is obnoxious, so I’d recommend 48 steps or more. Goldpoint and Khozmo (Poland) look like nice options here, for a reasonable price. You can really kit out a custom Khozmo to your specs, and they have remote-control options. Every few months I think to myself that I could use one of those Khozmos; it’s a wonder I haven’t ordered one yet. Little channel imbalances drive me crazy!
What I'd REALLY like, ideally, is an RK50 pot (that's the big expensive one) with motorized remote control (just like VAC implements in their Master and Statement preamps) AND magically with the split-dial like Stax used to do for their headphone amps. One rotation adjusts main volume, and the other affects L/R balance - while riding atop the main level.