Power of audio amplifier not matching specifications
I fed in a 50 Hz sinusoidal signal, and turned amplifier up until right before the -20 dB light came on. 


What you've measured is the power output 'right before the -20dB light comes on'. What makes you think that light is correctly calibrated, and what level do you think it's supposed to represent.

How about you measure the actual maximum power output? Note, do not attempt to measure the maximum power into a multi-way loudspeaker, the onset of clipping may burn the tweeter out, use a dummy load.

There are several ways to define the maximum output power. The usual criterion is some measure of distortion, either visible waveform clipping on an oscilloscope, or a measured THD on a sound analyser. If you have neither of these, then a measurement of output voltage will give you some indication of when you've got to clipping. If you're measuring peak voltage (you've built a peak detector), then at clip the output voltage stops increasing as the input increases. If you're measuring mean rectified (cheap DMMs) or RMS (better quality DMMs), there will be an abrupt reduction in slope of output to input voltage gain at the onset of clip.

Another way is to do the same thing, but with bursts of tone rather than continuous power, which is said to represent musical dynamics better. This requires a suitable generator as well as an oscilloscope. It usually results in a larger measurement than for continuous power, and there is much debate over whether this is a 'fair' number.

Another way is what's the maximum power that can be generated for X time before the amplifier overheats. If the designer has done his job well, this is larger than the distortion limited maximum power.