DIY Distortion Analyzer/Analysis?

The Cordell distortion magnifier is a pretty good tool. With modern op-amps that same idea, plus a halfway-decent sound card should have no problems measuring the distortion of even a SOTA power amplifier.

Alternatively, here's an interesting option:
I've had very good results with a Topping D10s and the Cosmos ADC, with an Autoranger in between.

This post is what inspired me to purchase the D10s: PMA Measurement system