Thanks for your help. Since I don't have a "modern day AVR", I'd have to find some sort of media player set up with 4 channel analog outputs.
Nothing you buy today will offer only 4 analog outputs. 4 channel Quad is a dead format. Anything modern will have 6 or more channels. Nothing says you have to use all those channels though.
Below I am speaking only about the playback chain.
A typical media player will do what you are asking for stereo. Most are equipped with a stereo analog out jack where the device will convert the PCM signal to analog for use by your amplifier. One fellow I work with uses a system like this. I helped him set it up. A refurbed, fanless windows based media player cost him about $200. A 4TB HHD cost him another $100. A blue ray burner/reader was another $100. A Logitech wireless keyboard controls it all. With it, he can rip all is CDs, store them, and play them back through his vintage Macintosh tube gear. He uses JRiver Media Center (software) to do this. He loves it.
But a stereo output wont help you for quad playback. There are solutions however.
The easiest is to purchase an Oppo 103, 105, 203, or 205. These can only be had on the used market since Oppo is no longer manufacturing them. Your digital files would be stored on a small USB storage device that plugs directly into the Oppo. This could be a portable Hard Disc Drive (HHD) or Solid State Drive (SSD) for larger libraries, or a simple USB thumb drive for smaller libraries. The Oppo provides a rudimentary navigation routine for choosing files, does the digital to analog conversion, and outputs up to 8 channels of analog audio. Many forum members use this method. With a 103 or 105 you could also rip your own SACD disks for playback (very desirable). Oh yeah. It also plays optical disks of pretty much any flavor. The down sides are:
1) Used Oppos are not cheap, especially the X05 models. 103's aren't so bad and go for $350-$450. That's pretty close to their original cost when new.
2) The user interface is very basic compared to dedicated software for playing music, but it may suffice for you, especially if you have a smallish library.
Here is a another possible solution for you.
This device accepts a PCM digital signal from a USB port of a media player and decodes it to analog. You'd still need a media player, a storage device (HDD), and some means to control it all (keyboard or remote). A setup like this allows you to use pretty much any player software you like such as Kodi, Foobar, JRiver, MusicBee, etc. All provide a quality user interface. This solution does nothing to allow you to copy recordings like SACD or blue ray disks, let alone your legacy reels. This is an inexpensive DAC solution @ about $250. More expensive units are available on the miniDSP site that include room correction via Dirac Live.
Both solutions require a video screen for control.
Most of these media players will work. There are many others. Many that use other operating systems beyond windows.