Almost anything played on my 2nd systems, Unitiqutes. If it’s on my main system, NDS with active Ovator s600, I have to stop and listen.
I should say the dealer (UHES) I had when I only had a main system suggested to me that I should consider 2nd systems as a background source and not for serious listening.
If I want background music I just turn the volume down. There are some bands/singers who probably aren’t ideal as background music at any volume but most music I believe can work in a different way played at a lower volume.
The only purpose I can see of background music is to mask undesirable noise that otherwise would be intrusive or distracting. For that any pleasant but not interesting music of limited dynamic range would do - but what fits with that for one person likely wouldn’t for others.
I find vocal music impossible as background, but instrumental is fine.
From the classical oeuvre, I find Brahms particularly good as background (to the extent that I find it difficult to enjoy as ‘foreground’ music, but that’s another discussion).
From jazz, Bill Evans’ gentle style is excellent.
From pop and rock… not so much. Too many vocals, everything’s too much in the foreground.
I also enjoy background music while reading or working from home. Shakti at low levels works as background music, but also bears up to critical listening.
The stream from the local public classical radio station (WQXR in NY) works too.
To my ears, all the music I like - rock, prog-rock, classical, tragic opera etc - is easy listening! The “genre” of “easy listening” music, from what I have heard of it, is music to either cause either instant sleep or trigger a desire for rapid anaesthesia if not euthanasia!
I think Internet radio is ideal especially jazz and classical, and RP Mellow Mix.
It will play all day.
Some easy listening stations can be chosen and periodically you can swop stations for a little variety.
Background to what? First things first - I am a retired lawyer, so work was mostly reading and writing. I continue to read and write in retirement.
Writing: Classical guitar is the most reliable. But Baroque music (non-vocal and not Bach) and classical era chamber music often work.
Serious reading - Classical guitar is best. When maximum concentration is required, I have to turn off the music entirely.
Light reading (mysteries and such) Almost anything classical or jazz other than English language vocal music.
I also love watching baseball with the TV sound turned off and almost any non-vocal classical music turned up. Haydn is probably choice No. 1, partly because he composed so many enjoyable works and for me he fits the baseball season.