The 804's are not an appropriate speaker for loud listening - they are like a small bookshelf speaker, with an extra small driver. Based upon what you've written, it sounds like you've damaged a driver or the crossover in one speaker.
If I were you, I would have both speakers checked for damage (the speaker that sounds normal has been experiencing the same stresses), get them fixed if they need repair, and sell them. If you want to do loud volumes, you should buy a pair of used WattPuppys or powered speakers like ATC (horns are also excellent for high volume, but they require a fair amount of knowledge to use properly).
If I were you, I would have both speakers checked for damage (the speaker that sounds normal has been experiencing the same stresses), get them fixed if they need repair, and sell them. If you want to do loud volumes, you should buy a pair of used WattPuppys or powered speakers like ATC (horns are also excellent for high volume, but they require a fair amount of knowledge to use properly).