What Alan said. You can put a resistor on your kitchen table and measure the voltage across it as a result of intrinsic and extrinsic noise. Intrinsic from thermal fluctuations and extrinsic from stray fields.

To answer your question, a signal to noise ratio of over 85dB referenced to a 1 Watt output into the rated load with the inputs shorted and measured over a band from 20Hz to 20Khz at maximum gain is a figure your ears won't object to.

If you are really concerned about the noise, drown your amp in liquid nitrogen.

Does that help smile?

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated