Source for extent of compression for CD's

Sorry - there is no easy answer or way to measure/define this on CD's, labels. In terms of speakers, go for designs with pro type drivers.

I doubt that the classical labels use much or any compression for CDs. The dynamic range of CDs makes compression technically unnecessary EXCEPT as a deliberate means of making the music loud which is a pop music thing.

Several labels (I forget which ones) state that no compression, equalization, or other processing is used.

LPs need to use peak limiting and compression to avoid stylus mistracking, particularly when the LP is played using a less-than-audiophile cartridge. Compression may take the form of "gain riding" which is done manually by a recording engineer familiar with the music. And finally, compression may be done by the performers who simply minimize the variation of the loudness of their playing when making a recording.