How do you determine how much to spend on speakers

The speakers are a choice that is left up to a persons own ear

For that matter, so are all other components. I'd add the "eye" as well as the ear.

In practical terms, however, the above statement indicates absolute precedence to the speakers: speakers are the ones coverting an electrical signal (that we do NOT hear) into sound pressure (that we DO hear).

I agree with this take on the statement above (even though it doesn't seem like the intended one:)).
In ABSOLUTE terms, you choose a speaker and then work upchain accordingly. In real life's relative terms, things aren't so simple: pbs such as, i.e. "how do you drive said spkr" come to mind.

Maybe a spkr+amp combo is the way to go! I for one would strongly urge in that direction.

If the music was properly recorded (than) then it is going to sound good when it is reproduced despite your equipment

IF the music was well recorded. But usually one buys the music and the musicians playing the music -- not recordings.

What in your opinion are the best bookshelf speakers for classical music

Fred -- that's a difficult question to answer. The BBC type speakers maybe? Or monitor types like the ATC (great midrange driver!). Maybe a Sonus Faber...
There was only one small B&W I liked called something or other "silver" (which doesn't help much, sorry).

Goebel is worth suffering for:) BTW, a have a few Koln cd's (more on vinyl) -- but they're not that bed sounding (true, a bit shrill). Cheers