I think your question might be interpreted as some form of speculation, which is something not encouraged in this subreddit. However, I think it can also be read as a question about the differences between the instruments of the classical period and contemporary ones (which have not changed that much since the late 19th and early 20th century; I am leaving new instruments out of this).
Does classical music sound different with new instruments? Yes, it can sound noticeably different.
Instruments have changed. We are certain about that because there are some surviving instruments (plus depictions in art and mentions in texts).
You can listen to one of Mozart's pianos in this video. That piano dates from 1781-1782 (Mozart died in 1791), and it is now part of the permanent exhibition at the Mozart family home in the Austrian city of Salzburg.
I recall listening to one of Haydn's pianos in BBC's "The Birth of British Music, Haydn."
Going a little forward in time, here's the first chapter of a short documentary about one of Beethoven's piano, a Broadwood from 1817. You can also listen to Beethoven's Graf from 1825 in this video.
That kind of instrument is called a fortepiano. They are quite different from the modern piano in several ways, you can see a comparison in this video.
What about other instruments? Yeah, there were differences, too. Wind instruments changed in the 19th century, better (more complex) systems of keys were added, they became louder, and had more agility and better intonation. Brass instruments got valves, and were now able to play more notes. Timpani were different in classical times, smaller and with a different sound. The difference in bowed strings is less significant, but they had slightly different bows (and the violin had no chinrest).
Do you want to know what their orchestras sounded like? Without a time machine, the best I can do is point you to some orchestras that use period instruments or replicas:
The differences would not only be in the instruments used, but in many other details of performance practice.