What I'm looking for is a box of tricks which I can connect to the line in on an amp and play back music
This is one of the ways I listen to music, with one of these
(Sony NW-A3000)
or these
(Philips HDD120)
or these
(Creative Zen Touch)
plugged directly into the amp through a 10m 3.5mm jack -> amp inputs lead.
You have fingertip control (keep the player on the sofa), and can skip, rewind, ff, adjust volume, whatever - and the sound quality is really very good.
the Philips one (the second, above) actually makes high quality recordings to MP3 as well!
All these players come up on
ebay constantly, and can be had for as little as a tenner, including delivery. They're 20gb, and hold loads of music - certainly the kind of shopping list you describe would be no kind of challenge. I just checked the one I have here on my desk, and it reports 2,912 tracks.
Battery life tends to be poor - no more than two or three hours, typically - though you can buy and fit replacements, which push that up to 10-12 hours. But since they live on the sofa, that's not really any great hardship.