elevating subwoofer...

Yes large steel balls would dissipate energy. Can you find them easily?

It is easy to find replacement spikes; buy a couple of bolts and check the thread and then buy the spikes. Parts Express or other suppliers will have them.

Why do you need spikes for a sub? Unless it's downfiring of course.

Your factory spikes are likely either M6 or 1/4-20 threaded. Go down to Home Depot and buy a bag of each thread, see which one works with your inserts. Then buy some replacement from any of the many places around, like Parts Express ...