FC vs. AV FC Audio Question

So the extra audio channel is not mixed properly on the AV Famicom, i.e. the extra channel is much louder than the CPU audio, but is mixed correctly on the old version FC. I've heard it myself on Akumajou Densetsu (CV3). Has anyone tried replacing the AV Fami CPU with the one from the FC to correct the issue? I have parts laying around to try this. Just wondering if it's a waste of time.

An easier solution would be to find the right set of resistors for proper audio balancing.