1. All the formula are available in the linked site, but if you have trouble understanding them or not sure how to use them just let me know.
Calculator damping loss and dB amplification factor gain decibel amplifier audio engineering microphone recordings calculation ratio - sengpielaudio Eberhard Sengpiel
www.sengpielaudio.com
So for the specific example using 1.4 V input and you want to get 400 W output, the math would be as follow:
First, find the required output voltage required for 400 W using the power formula,
P (for a resistor load) = V^2/R, or V = square root of PXR
V=sqrt(400X8) = 56.57 V
Then calculate the Input voltage required for an output voltage of 20 V using the formula:
Vg (voltage gain in ratio, that is multiples) = Vout/Vin
or Vg = 56.57/1.4 =40.4
Now that is a ratio, to express it in dB, the formula is:
Vg (in dB) = 20*log10(Vg in ratio)
Vg(in dB) = 20Xlog10(40.4) = 32.13 dB
2. You have to ask Purifi, as it depends..