Nice thread to resurrect. IIRC, Readers Digest, among others, offered gear somewhat as an add-on to their regular products. Certainly, customers who were comfortable with RD's financing and mail-order purchase processes would have been quite compelled to continue that relationship.
Back when these items were being marketed, there was a start-up wave going on in Japan of contract, build-to-spec, manufacturers - we would call it outsourcing today. Many of these were actually subassembly suppliers to all the big names we recognize.
They would advertise their abilities to deliver sample receivers and amps in the back of many of the trade magazines servicing the fast-growing consumer electronics industry. You'd see small ads for "Send us your performance specs, front-panel layout and we'll get you samples in less than a month; full container loads of product in less than 2 months from approval!"
Of course, this arrangement meant that the 'owning' brand in the US would have to take delivery of the product, write an owners manual, stock spare parts, create boxes, build a distribution network, and all of the other costs of bringing a product to market.
Readers Digest already had many of these requirements in place with their other products so that cost of entry into the consumer electronics market was lower than most.
Today, we see similar tactics being played out by folks like Best Buy where their Insignia brand is pretty much just that - a name with contract suppliers delivering product to BB's specs, price point and profit margin projections.
Cheers,
David