Rear speakers not stock?

I open up my rear two speakers (only a 4 speakers system) and thye dont seem to be stock speakers, unless I am mistaken. They arnt made by delco electronics like my two front ones but another brand and they say 50 Watts on the back. They are a black colored subwoofer like speaker which is like a plastic color w/ a black cone. Are these stock? My whole system has no bass.