They dont have to be Infinity speakers.

 

I have put so many different speakers on stock HU it is un-real, anything from Best Buy, Circuit City wherever is better than stock so just throw something in there.

 

Like he said, for the most part. I didn't like the sound quality of the sony explod series. Alpine, JL, etc. Just try and match the RMS power rating of the speakers to the head unit. Also look at the sensitivity level (in dB I think). They tend to respond better to lower power levels. Oh and the ohm rating like mentioned before.