0dBFS 1kHz sine defines a peak value.
Alignment of audio levels is usually done at an RMS level of -18dBFS or -20dBFS depending on the standard you are applying.
The line level alignment level that the amplifier will use depends entirely on the manufacturer. For the sake of argument, let us consider that the alignment level is -18dBFS.
-18 dBFS = 0 dBu = 0.775 V rms (sine wave) = 1.1 V peak
As you are considering off-the-shelf amplifiers, the voltage and power amplification that is performed after initial alignment will depend entirely on the specifications of the amplifier.
Also note that this particular amplifier works on a PWM basis where the inductance of the speaker forms part of a low-pass filter.
"This amplifier is designed to drive moving coil loudspeakers only. Speaker impedance must be 4Ω or more. The output signal is a 330KHz PWM square wave with a duty cycle proportional to the audio signal. The inductance of the speaker coil serves as a low-pass filter to average out the high-frequency components."