The quote of this post's title is from "Why You've Never Really Heard the Moonlight Sonata" by Jan Swafford, in Slate. The first chunk is about Michael and Patricia Frederick and their library of pianos dating from the 1790s. The second chunk is about how the construction, and thus the sound, of pianos has changed over the years, and how the physical piano probably influenced the choices composers made. The final three paragraphs analyze why there's so little diversity in piano sound these days.
Listen to the audio clips for comparison! There's one of VCU's Dmitri Shteinberg playing an 1877 Erard piano.