We can expose the mythical 1400 watt amp another way. Watts are watts, no matter the voltage. If a lower voltage is used, more current is drawn to make the equation balance.
A common household electric space heater is 1500 watts. At 125 volts from the wall outlet, it will draw 12 amps. If the AC drops a bit to 120v during peak demand or if your home wiring is light, now it's taking a bit over 13 amps. Everything works harder and wiring heats more, because it's the current, not the voltage that has gone up.
Let's examine what's needed to handle 1400 watts in a 12v car electrical system. 1400/12=116.66 amps! Your typical home circuit breaker trips at 15 or so amps and you'll notice extension cords and outlets feeling quite warm when you run that floor heater for awhile. You can pretty much power a light metal-welding job with this much juice.
The tiny hairlike wires in a typical speaker's voice coil would vaporize if actually given this many watts. No benchtop 12v power supply will do, you'll need a bank of batteries and 00-gauge wire.
Even assuming the amp might get close to a truthful 50 wpc rating, it's still a big draw. 100 watts/12=8.333 amps current. Most experimenter's supplies run 2 to 4 amps, you'll need a 10 or 12 amp power supply to run this subamp. On the typical power supply, the subamp would simply flake out when asked to go full output since the ps could not keep up and would trip its breaker. Your car battery wouldn't start your car after an hour or three of this.
As Audioguru surmised, a true 25 watt per channel output is much more honest and realistic. And that's with distortion so high they intentionally leave out that spec and little in the way of protection circuitry for the speaker or ps. For easy math, we're also assuming 100% efficiency, 60% might be closer to actual use. My example is at full output. In use on music, that's a very flucuating number and maybe some big capacitors would help on the peaks to take the instantaneous load off the ps.
Heh, my second post here in 13 years, I'm on a roll!
-Ed