5150's are very controlled designs because of design complexity and their borderline verge of stability. Theoretically there is alot going on. It is nothing like the marshall attitude of changing something to yield a sound return.
a log pot would cost more than a normal potentiometer, James Brown knew what he was doing and accounted for this when he designed it to be both efficient and price-point secure.
Just changing the potentiometer itself would do no good anyway, but even if it did it would not yield the expected results, and trying to remove gain elsewhere without a domino effect almost impossible. James modified the logs on alot of potentiometers manually all over the amplifier since they are custom 16mm designs made for PCB mounts. As Mike Fortin once said - 5150's are terrible platforms to look for modification in getting more or less of anything out of them, it is better to start with something much simpler alone. To remove gain on only one channel is not simple at all due to how the amplifier is laid out theoretically. On top of all of this, the entire preamp has to be dissassembled and removed from the amplifier when doing so, which is not a task for the faint of heart and does take quite some time to do. It is also possible to damage the preamp board removing it if you're not careful.
Basically it is not worth it for something this simple, it will cause more trouble than its worth to attempt it unless you really know what you're doing inside there.