SNR at 1W can be calculated from SNR at rated power. This is not just for Cherry Amplifier (R) products, but for any amplifier.
If we know the signal level and the SNR, we can calculate the noise level. This is typically A-weighted, by the way, but not important to the math....
SNR is the ratio of signal to noise, or signal/noise. These are voltage levels. Units must match, and we use volts. SNR = 20 * log(signal/noise). Since there is more signal than noise, SNR is positive. If it were noise/signal, the result would be negative, due to the log function.
We start by reversing the log function to get the actual ratio in straight scalar units (no units, actually). So, we're "un-doing the dB", going from exponential scale to linear scale.
Let's say SNR is 120dB (MEGAschino, Maraschino). This means 6 = log (x), x being SNR as a linear scalar. 10^6 is 1,000,000. So, if the signal is 1V, the noise would be 1/1,000,000 V, or 1uV.
Let's also say the rated power is 400W into 4 ohms (KING Maraschino). Power is V^2/R in this case, so V = sqrt(1600V^2) = 40V. So, in this case, noise is 40uV.
Now, what is it at 1W (into 4 ohms)? Well, we know the noise voltage, and 1W is sqrt(4V^2) = 2V, so SNR = 20 * (2V/40uV) = 94dB.
The reverse also works, so if you know SNR at 1W, you can calculate it for rated power. Almost every amp manufacturer states SNR at rated power, by the way. Thanks.