Help My Nautilus B&W 804's 'pop' at loud volumes.

When listening to loud volumes for long periods of time (around 2 hrs), my left speaker has begun to make a loud 'popping' sound during a stressful point on the speakers (example: Explosions near the end of 'The Dark Knight' Batman movie, or a tight bass hit during play of the new music Blu-ray version of Rush's 'Moving Pictures'. There's no evidence of a tear in the speakers dust caps, cone or their surrounds.

It doesn't seem to happen earlier during my listening experience. This leads me to believe it may be Amp or 'heat related'. But I'm still curious why it only happens to the left side speaker.

The speakers are driven by an Anthem AVM-20 pre/pro and MCA-50 amp. I'm using RSC Prime BiWire speaker cables by Tara Labs.

What do you consider to be loud?
Popping can be an amp clipping . If possible try a friends amp , see if the same thing happens at the same DB level . Have you tried swapping the speakers or speaker leeds from left to right ?
Clipping or speaker damage...all speakers get very hot inside the voice coil and then sound dull - you may want to look at pro speakers if you want to retain dynamics at high SPL.
I had the same problem years ago. Turned out to be something in the amp. The mfg had to replace some parts. I agree with Tmsorosk, see if you can try a different amp to help isolate the problem.
Good luck, John
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.
I agree that the pop may be from your amp clipping. From what I can see on my 804's is that they are total power hogs. I have a Mac 7150 (150 watts per ch) and I pegged the meters once and it wasn't all that loud. So unless you have a monster amp on those babies, you may have to lower your level.