Yes, because impedance is different at every frequency. Go ahead and try a bigger amp if you like but it may not translate into a whole lot more output especially if you haven't got electrical system to support it. Perfect world, going up to the 8K amp may give you about 1 more click on the volume knob worth of output.
No it isn't and no it doesn't. For reference, z-max at Fs (47hz) is 6.25ohm free air, Z at 47hz in the box is 3.5 ohm and Z-max in the box is 6 ohm and at 30hz. Free air impedance at 30hz is 2.05 ohm.
It would be very poor practice to count on such generalizations if you were attempting to put yourself in a position where any of this is important. IF you really needed to know your impedance curve you must measure it in box in vehicle. This can be done with some graph paper or spreadsheet, tone generator, and DMM or O-scope or a tool such as Dayton DATS.
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