The "20Hz at -3dB" spec indicates that at 20Hz the speaker or subwoofer is only 3dB quieter than all of the higher frequencies - where we assume it is relatively flat in frequency response.
The "30Hz at -6dB" spec indicates that at 30Hz (a note 1/2 octave higher than 20Hz) the speaker or subwoofer is 6dB quieter than all of the higher frequencies.
So the "20Hz at -3dB" spec'd speaker plays considerably lower.
As for the amplifier spec - a "perfect" amplifier will output twice as many Watts every time the impedance (Ohms) is cut in half. So yes, if that is a "perfect" amplifier, it will output 50 Watts into a 4 Ohm load if it is spec'd to output 100 Watts into a 2 Ohm load.
However, very few amplifiers have this sort of "perfect" doubling of Watts into half the impedance. In most cases, as the impedance of the load goes down, the amount of noise and distortion goes up. So if all specs are kept equal, you will usually see fewer than double the Watts when the impedance is cut in half. In other words, an amp will often be spec'd for something like 200 Watts at 8 Ohms, but only 300 Watts at 4 Ohms. If the amplifier were "perfect", it would output 400 Watts at 4 Ohms in this case. But if the noise and distortion specs are being held constant, you'll most often see the Wattage increase by less than the "ideal" double when you cut the impedance in half.
It's a bit rare to see an amplifier spec'd into 2 Ohms. I'm guessing that's a car stereo or subwoofer amplifier. Going from the lower 2 Ohm impedance to the higher 4 Ohm impedance should be no problem. You should get 50 Watts at 4 Ohms no problem, and quite possibly even higher if you're keeping the noise and distortion specs the same as the 2 Ohm rating.
Hope that helps!
