What do you consider to be loud?
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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).
As Raquel stated, get your speakers repaired if necessary. You didn't say, but it sounds like you are not using a sub for the bottom end. If you aren't, you should consider getting one (or two) to reproduce the bass frequencies. If you don't change your listening habits, or get speakers that are made for that kind of punishment, you will continue killing speakers.
Sounds to me like you may be bottoming-out the voice coil. A loud transient can push the speaker out pretty far and the recoil can cause the piston to hit the motor assembly.
It would be worthwhile to verify that the speaker polarity is in phase with the amp. In other words that the positive output from the amp is connected to the positive lead on the speaker. You can watch the cone to make sure that it is pushing air on the initial attack of a note. If it seems to be moving inward on the initial attack you are more likely to bottom out on transient peaks and damage the voice coil.