I recall as a young man going to the drag strip and seeing the rocket and jet cars. Visually, stunning, audibly hellish. I was too stupid/young to be prepared for the assault on the ears. I ran to the bathroom and jammed as much toilet paper into my ears a possible, then sat the entire time with my finger pressed against the outer ear to close the auditory canal as best I could. Still, it was awful. It's audibly like staring into the sun.
I have always detested loud noises and sought to reduce them in life. I do not recommend people go to many live concerts where there will be potential for hearing loss. In the last 10 years when I have attended, perhaps 2 or 3 times, I used both foam 30dB plugs AND ear muffs. Still, the level at the concerts was irrationally loud, and even with that protection had mild tinnitus that evening, gone by morning. Imagine the damage being done when all that protection is used and still harm occurs.
I also wear both foam plugs and muffs when using the lawn tractor. Finally, when I ride motorcycle (I intentionally do not ride a bike with loud pipes), in order to protect from the hellishly loud wind noise at freeway speed, I wear the foam plugs and bought a very expensive helmet that had the highest noise control rating. I am hopeful that all these measures will preserve my hearing longer in order to enjoy music.
We are going to have a generation of prematurely deaf people. It's a shame, as hearing is one of the miraculous gifts God has given and it's desperately wanted when it's gone. I say all this not to chastise those already suffering; they know the price that is being paid. I say it to encourage anyone younger who has been taking their hearing for granted, abusing it without thought.
In an effort to be helpful to dougeyjones, I haven't seen anyone mention speaker placement as a panacea. You have both placement and toe in to work with. I presume you have tried all that? If the speakers have grills you could perhaps try taking the grill off the side with the hearing loss. Experiment.
Another idea is one that you may scoff at, given how you disdain aftermarket cables. I suggest you try doubling the speaker cable to the affected channel/ear, and in so doing improve the output of the speaker in that channel. I learned this once when I forgot to remove the second speaker cable from one channel and the sound was imbalanced. That may be a golden solution, however, it does also affect the speaker tonally. But, the fix may be better/less noticeable than the other artifacts as a result. Perhaps if you are desperate enough you will take a tip from a cable enthusiast. :)