The other day I was testing my F1 subs, making sure they were fully out gassed (they seem to be) and I had the volume cranked! I had the volume on max on the F1 subwoofer and max on my mixer (Denon X900), the lights on the back of the subs were solid red and the limit lights were on and the subs still sounded good. I decided to see how much more I could push the volume... I wanted to see if at any point I could hear distortion... In which case I would back off the volume. Please remember I was also trying to get the smell to return from the subs....that is was the original reason for testing them (not trying to damage them in any way...) and they say to hit them hard with lots of volume with deep low ended tracks...
** It is important to note that at this time I had the -10 attenuation switch on on the back of my mixer. Also, I was playing some extremely bass heavy track by Bass 305 and Bass Boy.
*** It is also important to note that during this test I did not have any F1 tops hooked up, just the subs... This made it easier to hear if sounded like they were being over driven...
I repeated the test with the -10db attenuation switch now set to off and slowly turned all knob to max (including the trim) all pfl meters were red everywhere and still the sub sounded great!
I was amazed at the internal limiting processing of the F1 subs... Here I hit them with an additional 10db and they never faltered! You could hear at a certain point the subs topped out, but again the internal processing kept things clean no matter what I did! 
My conclusion, the F1 subwoofers are truly bulletproof!!!