Setting up a bypass test can be pretty easy if you have a tape loop (which is how we used to do it in the old days). You merely put a preamp (for instance) into the tape loop and then switch it into and out of the signal chain. As you can well imagine, very few components ever completely passed JGH's bypass tests.
About ten years ago David Salz of Wireworld designed and built something he called "The A/B comparator" for doing bypass tests on cables. It even had an X setting where it chose whether you were listening to bypass or the cable in the circuit. I had one that I used for a couple of years before I returned it to him. It made me crazy and proved that cables did, indeed, sound different.
While the goals of high-performance audio are the same as they were 60 years ago - to do as little to alter the harmonic, dynamic, and spatial characteristics of the original signal as possible, the way that is accomplished has changed over the years. And while some might argue that we have only gone sideways instead of forward (which I would dispute) even the most contentious audiophile or audio critic can agree that we still haven't gotten "there" yet.