Polk Audio Monitot 50s

Manufacturer or authorized factory repair center for Polk.

carefully prize of front cover

sq nearest expert radioinstaller I'm sure he will know the answer and have the replacement part you need

Not all surround modes use the center channel. Try changing mode on your receiver.

it sounds like its the cross over

The front panel is pressed into the cabinet. Take two medium flat head screwdrivers and put them in the top of the speaker in the groove around the front speaker panel and gently pry the panel out. There is no silicone or adhesive holding it in,it is just an extremely tight fit.

eBay is always a good place to start you can get anything, the problem is replacing the driver in just one will give uneven sound as it would probably be dificult to find one that matches, i would advise replacing both, it will need to handle 160watts max and go down to atleast 75hz the crossover will take care of the high frequency range so this is not as important but it should be bale to handle atleast 8khz (8000hz) and pay close attention the the resistance of the speaker, it will probably be 4 ohms but could be 2 or 8 depending on how its wired, you can check by connecting a multimeter set to 100 or 200 ohms and connecting it to the 2 terminals on the speaker (after its taken out of the cabinet) the meter will read 0.0x where x is the resistance nearest to 2 4 or 8.

polk dont supply spare covers best you can do is have a carpenter build you a new one

Passive Radiators are not driven by the amp. They have no voice coil and no power goes to them. They are used to enhance the bass or lower frequencies in a speaker system. They work by utilizing the back pressure that a driver (woofer) produces. When a woofer produces sound, it moves forward and backwards inside the speaker cabinet. This backward motion is what drives the passive radiator. So in order for the passive radiator to work, it would have to be placed in speaker cabinet along with a woofer that is hooked up to an audio amp.

http://www.crutchfield.com/brands/Polk-Audio/
You might give these folks a try.