did you look at your speakers, or are you just going by past posts?
I just replaced speakers and head unit in my 2011 Versa S with base radio. My factory speakers are 4 ohm, cheap paper/fabric. I have pictures, but don't see a way to attachthem.. maybe because it's my 1st post. I won't knock them too much since they work great for a aftermarket speaker mounting brackets, once you rip out the speaker bits. I even soldered the new speaker wires to the tabs on the bracket, so i just plugged in factory speaker wire plug back in.
Also, pay attention to wire colors. The "ground wire is towards front of the car" doesn't work on 2011 Versa either. well, maybe on just one side of the car
Since you already have the new head unit, I would replace the speakers..
If you don't want to destroy your factory speakers to mount new ones, i've seen nice looking mounting brackets/spacers, but they are $20some a pair... but then you'll be cutting your factory speaker wires to splice into them..
My system: Pioneer DEH-6300UB radio, TS-A1673R front speakers, TS-A1643R rear speakers. I used Metra kit and wiring adapters.
As others said many times, try to listen before you buy.. If I did more research before placing my orders, I probably would have gotten the Polk 651 speakers.. I got to listen to both 651's and 1673's at Fry's and the Polks sounded little more... well rounded? to me, at least for some music..