"We never sit down and listen to a music record through an amplifier in the design stage".
There is nothing unusual about this!
It is about testing what you have designed meets the designed specs, i.e. gain, bandwidth, distortion, noise, signal to noise, circuit stability and of course CE regulations at this phase of the development. Further tests would be to see if any variation in components or alternative manufacturers parts would effect the results or design. When the designer is happy with this the next phase would be to see how it interacts with other equipment (including other manufacturers electronics) and this is where some listening tests might come in. When all that has been done the designer, or owner of the company might want to "voice" or fine tune the unit to get the best result. This is more typically done today. There are a number of other tests that would need to be done, assuming what was being produced was going to different parts of the world...
Don't forget that when Peter Walker was involved with QUAD there was very little choice in what parts could be used. It has only been in the last 30 years where great advancements have been made in construction of electronic components, where some have been specifically designed for audio use.