I have all sorts of test equipment. I've been using my tone generator at about 1Khz, 0.2V amplitude, and right now the waveforms look fine when I connect to main in, although the left channel is about 1/10 the signal strength of the right channel. No distortion or harmonics though.
I have pretty close to every piece of test equipment you could need for this job, so you don't have to think of ways to make things workable for the average person. Oscilloscope, tone generator, radio communication test set that does AM, FM, and FM Stereo, dummy load, speakers, variac, isolation transformer, DBT, large DC power supply, soldering/desoldering station, wire wrap tools, enough electrolytic caps, transistors, resistors, ceramic caps, etc., both through hole and SMT to choke an elephant. I even have a microscope, tweezers, and hot air if the need arises. lol
No distortion meter or curve tracer though. Also I never have enough cables, but I have a wall full of them.