I'm not sure. Adjectives like warm (or maybe smoother and less brittle) usually mean less high end, but I don't think that's all there is to it. I think part of it may be good mastering without overcompression or clipping, and good instrument tone and arrangement (and probably equalization) - with discernible bass, midrange, and highs (though cymbals and high end can get sanded off with repeated plays I guess.) Better playback systems (vs. bluetooth speakers etc.) may also be helping. The number of tracks and stereo image might be helping. And maybe there's some kind of synergy between vinyl-era popular music and its recording media - bands that sounded good on vinyl probably sold more records, etc.

On the other hand, I am sometimes blown away by the dynamic range of good classical CDs.

Also: has FM radio gotten worse in sound quality?

> I’ve always said that vinyl sounds “warmer” but that is a squishy term that I can’t really elaborate on.

I think it is at least partly due to a reduced top end/treble. Though googling something like "why does vinyl sound warmer" will give more information. Sites like ASR forums or diyaudio will help if you want to get more technical.

A decent amplifier and speakers probably. People tend to spend money on their vinyl setup. CD played through the same setup would also sound good.