Bypassing factory amps.

Make sure the speaker polarity is correct.  Oftentimes when less bass than is expected is heard, the speakers are wiring backwards (negative to positive, positive to negative).

As for the speaker impedence, a speaker with a lower impedence will be louder than an identical speaker of higher impedence.  This is because it is offering up less resistance to the amplifier's output.  I'm surprised that the jensen deck says you can use 2 ohm speakers, but hey if it says that I say go for it!  But yea higher impedence speakers will not necessarily give you more bass, and they definitely will not be as loud.  But I guess it's worth a try if you have extras lying around to test with.