I just got my new Pioneer DEH-590IB deck today. The instructions say that it "should" not be used with speakers below 4 ohms.
I'm using 5.25 components up front with 2 way 6x9's out back. Both these speakers are rated at 2.65 ohms.
Pioneer radios have very robust power supply and can handle speakers down to 2ohms without a problem. I would recommend using an amp for the components at the very least. Components are usually not real efficient and your volume level will be much lower than the 6x9's. An ideal situation would use a 4 channel amp for all 4 speakers and would give you plenty of power to get what ever volume level you want easily and allow you to balance out the fronts vs. the rears.
I'd suggest a 4-channel amp around 100watts/channel RMS that's stable down to 2 ohms without bridging. Taking a quick look at Crutchfield, something like Infinity Reference 475a, Alpine MRP-F450, or Kenwood Excelon KAC-X40. Any of those three should sound great and let you crank it up without damaging the speakers (the distortion from low-power amps is what usually blows speakers).
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