The signal-to-noise ratio is a rather important parameter and describes how much noise or hiss the amp makes. This noise is produced by the amp itself. Make certain that the gain of the amps is set to the same amount. If you prefer an amp with a small amount of hissing, you can look at the signal-to-noise ratio number of the specification sheet. There are several reasons why power amplifiers is going to add some form of noise or other unwanted signal. Transistors and resistors that are part of every modern amplifier by nature produce noise. The overall noise is dependent on how much hiss every element generates. They are referred to as “class-D” or “class-T” small stereo amplifiers. Switching amps include a power stage that is continuously switched at a frequency of approximately 400 kHz. However, it may still contribute to loudspeaker distortion. Signal-to-noise ratio is usually only shown within the range of 20 Hz to 20 kHz. For that reason, a lowpass filter is used when measuring switching amplifiers in order to remove the switching noise. Producers measure the signal-to-noise ratio by setting the amp such that the full output swing can be realized and by inputting a test signal to the amp which is typically 60 dB underneath the full scale of the amplifier. Then the noise-floor energy is calculated in the frequency range between 20 Hz and 20 kHz and compared with the full scale signal energy.
Another convention to express the signal-to-noise ratio uses more subjective terms. These terms are “dBA” or “A weighted”. You will discover these terms in many amplifier specification sheets. In other words, this method tries to express how the noise is perceived by a human.