Well, here we are in 2018 and I have a cunning plan. It doesn't quite answer the question as posed but I hope it's close enough.
I have an old HTC phone that has a micro SD slot. 128GB SD cards are available on eBay now for a handful of $ / xc2xa3, so, objective #1 is to find out if this phone (it's Android 4.4.2 I think) can utilise all the space. [Update: it can, woohoo!].
Then I will copy my entire music library across from my PC.
Next up is a Bluetooth audio receiver that will pair with the phone and that you can simply plug into a spare input on your amp. Again, these are incredibly cheap on eBay. I don't know yet if they are any good, but people have posted some enthusiastic reviews so I couldn't resist buying one. It's in the post, will report back.
Then, install the music player of choice on the phone and to start streaming just press (I believe) the 'pair now' button on the receiver and then fire up the app. The phone should then play through the receiver to the amp.
- Nice and simple.
- Dirt cheap, if, like me, you already have an amp and speakers and a suitable spare phone. Even if you need a phone, it should be easy enough to pick up something second-hand, you just need that SD card slot. Or you could use an old tablet, but that would obviously be bulkier, clutter the place up more.
- No monthly fees (unlike, say, Spotify Premium).
- Should be easy to use.
- Choice of music players, pick your favourite.
- Cheap to run - no need to leave the PC on.
- Also works for Internet radio, podcasts and YouTube music videos. And indeed any other streaming service, most likely.
- Should also work from an iPhone (my partner has one, I'm hooked up with the wrong woman, what can I say?)
- I don't yet know how good the sound quality will be, it depends entirely on the receiver. But if it's not good enough I will buy a more expensive one. I'll let you know.
- 128GB might not be enough for your entire music library. You can get larger cards at modest cost on eBay but I think it's wise to avoid the very cheap ones.
- If you have a second music system in another room you would need to buy a second receiver. But they cost peanuts so it doesn't matter.
- Unlike some WiFi devices, such as (I believe) Chromecast Audio, you can't stream the same sound source to several receivers at the same time. But I don't care about that, and those devices seem to be focussed on (paid-for) streaming services anyway and I that's not what I want.
- Have to charge the phone or keep it plugged in. Maybe I can find a cradle for it.
So, a few unknowns still but I have high hopes. I'll let you-all know how it goes, should be fun.