is there really something unique about 6th symphonies?

[Madiel]

..and sometimes the psychology works in reverse. When Shostakovich produced his 9th symphony, he rejected the expectation that he would produce a Russian counterpart to Beethoven's 9th, and instead composed a sarcastic, neoclassical work.

Correct. But reverse psychology is still psychology. Subverting expectations still relies on there being expectations. People constantly comment on the nature of Shostakovich's 9th precisely because they know what a 9th is "supposed" to be.

[lunar22]

And yes, Prokofiev's Sixth is a great work, as is the wild Sixth Symphony of Karl Amadeus Hartmann!

Yes -- the Hartmann is another worthwhile mention. I remember Martin Anderson of Toccata Classics once saying it was a particular favourite of his which to him was the sort of thing contemporary composers could be aiming at. As a result i did have a listen or two and found it certainly a powerful work.

[AnotherSpin]

Any number will do. Is there really something unique about 5th symphonies? Is there really something unique about 7th symphonies? And so on.

[lunar22]

Could be and it's easy to say that. But I'm still waiting patiently for you to justify the 7th as an example. What is it about 7th symphonies which make them stand out from others in the series?