Is it recommended? Any harm done to the amp?

 

It can be done... I haven't seen anything personally bad happen from it. One thing to watch for it making sure the amp still has air flow over it. Also some amps come with rubber washers to help with vibration.

 

I've heard that amplifiers can be damaged this way due to the vibrations from the box. Something about internal parts becoming loose. I do not know how true that is though.

 

Yes there are a million tiny soldered connections in there that 'may' become loose but never seen it happen myself either so if thats where you want it i wouldnt worry about it. Main reason I try to avoid mounting on the box is ability to take the subs out of the car if i need the room.

 

That is the same reason that I have never tried it. I sometimes take the box out to make more room.

 

This mainly occurs due to poor workmanship. There is a way to reduce the chances of vibration loosening the components... but it's a long tedious process, and not worth the time.

 

rubber spacers work well...

 

i would use rubber spacers as well.

also, i wouldn't mount the amp onto the longest panel of the box. odds are it will flex more than the shorter pieces of the box, and the amp will flex with it. flexing amp = soon to be dead amp.