In clical music, are today's symphonies better/worse/equal to the symphonies in the time of the composers?

In athletics, the athletes get bigger, faster, better every generation, so a great player of yesteryear might not crack the starting lineup today. I'm wondering if the same thing applies to clical music - are instruments better, are techniques more refined, or is what we are hearing today pretty much what the composer heard in the 19th century when they first heard their work played?