They sold all manner of home electronics and their big thing was a catalog, which might have been monthly or quarterly. My uncle used get them sent every month. I think they predated Crutchfield and The Sharper Image. The owner would have these essays in the catalog that might not have specifically been about selling particular products, but as education for the consumer.
https://en.wikipedia.org/wiki/DAK_Industries
https://en.wikipedia.org/wiki/DAK_Industries