parallel vs series

It is true, if you run the subs at 4 ohms they will have 4x more power going to them.  That would equate to roughly a 6dB gain in volume.

The lower the load (impedence) an amplifier has to work with the more power it will output.  This is a function of Ohm's law that states Current = Voltage / Resistance.

To plug in some fake numbers assume your amps output voltage is a static 100VAC.  If your amp sees a 16 ohm load the amplifier will output 6.25 Amps of current which would be 625 watts (Power = Volts x Current).

Now, drop that load to 4 ohms and you'll see 25 Amps of current, which would be 2500 watts of power (exactly 4x more power).  Now, efficiencies of the amp will take a toll on actual numbers, but that is the math behind it all.

I may be wrong here, but I believe the only thing that will change with raising the impedence is you will lower the total harmonic distortion (THD).  So, with a higher impedence you will get better sound quality, but I don't think it will effect the resonant frequency as the resonant frequency of the car is determined by the car and the resonent frequency of the sub system is determined by the box.

You can't really express dB in a percent because it is a logarithmic function.  Without knowing your current system level it would be impossible to calculate the amount of a 6dB increase.

If we assume you are at 120dB and this change bumps you to 126dB it is safe to say that it will be a very noticable difference.

Really good information from KPierson, as usual.  All amplifiers (that I know of) will work with a 4 ohm load, so try the parallel wiring and listen for yourself for a few days.  You will know what you prefer.

By the way, running subs at 4 ohms is considered almost audiophile practice now.  They are most commonly connected to an amplifier at 2 ohms.  You may hear no difference at all in sound quality between 4 ohms and 16 ohms, but it will definitely be louder at 4.