Techniques for testing audio circuit designs?

I'm after some general information on learning/developing better techniques for testing audio circuits.

For now, (I think) my main priority is to compare the quality of the input signal with the 'dry' (unaffected) output signal so I can see what effect (if any) the circuit is having on the signal. An example of this might be putting a sine wave through a VCA to ensure that when the VCA was fully open - there was no change between input and output.

At the moment I switch between using a Sine Wave and a Square Wave and I compare the input and output signals visually on a scope...

...but if I wanted to get deeper than that - what should I be considering, researching or measuring?

As an example - my mixer specifications tell me it has <0.0007 THD. How is this measured?

Would learning more about FFT help?

I'd appreciate some advice on this - no matter how general.

Thanks!

you need a distortion meter,
something like this : https://www.akh.se/hameg/hm80/hm8027.htm
along with a pure sinusoidal generator like this : https://www.akh.se/hameg/hm80/hm8037.htm

these two modules can be found sometimes used on ebay.

also a great simple explanation about distortion meters here : **broken link removed**