Replaced the worn stock speakers in my Formula (4 speaker non-monsoon system) with Polk Audio units. I installed a set of DB6501 6-1/2" Component speakers in the doors, and a set of DB651 6-1/2" 2-way Coaxials in the sail panels. I run a Panasonic CQC-5210U head unit with no external amplifier, and these speakers CRANK out the music. I initially 'tuned' them to my ears and listening preference with the subwoofers muted, I set them with a good bit of mid-bass at a lower volume, and I was very surprised, as I cranked up the volume, the speakers kept up! Hardly any distorion all the way to about 80% of full volume, after that they fell off, but it was so loud inside of the car they will never see these levels on any kind of normal basis. My only complaints with them, and even crutchfields website said they wouldn't fit, but the only complaint I had was mounting them. They have the standard mount (4 screws) but they were just a hair small for the stock openings in the sail panel, so I had to modify with some self-tapping sheet metal screws. In the doors, they fit just fine, of course I had to drill holes in the door panels for the tweeters, and I was able to mount the crossovers near the top of the door frame behind the door panel, where they aren't stressed or anything.

Overall, Good product!