For instance, why does a Vandersteen, which I am sure is a fine speaker, cost tens times what my Polks cost?I can't answer on the price differential you mention & I have no idea which Polks you use & which Vander you are referring to, BUT, between a floorstanding Polk (9? It has 6 drivers, reflex loaded) & a Van 5, I must admit the Vand was much-much better for music.
* better high frequency, the V seemed to go much higher;
* better timbre, the Polk sounding shrill on a specific FR (upper violin notes);
* Van had bass extension and resolution vs the Polk stopping at midbass;
* far better resolution overall;
* the fee of dynamic impact was quite good on the Polks;
* Generally speaking, with the Vand I had musical coherency that I didn't have with the Polk. The Vand reproduced a coherent simulation of an orchestra, the Polk sounded small & tinny by comparison;
* Note, I listen to classical i.e. natural un-amplified instruments, so results could differ with other music.
AFAIK Polk is a very serious company, so the speakers I heard were probably designed for HT use rather than to play Mahler.
(The thing is, I'm one of the few people in the world who don't actually like the Vandy. Admittedly, they produce good speakers & value for money as these things go.)