What you put under your grand piano can make a huge difference in its sound. In designing a music room, whether or not it will contain a piano, I normally specify hard floor surfaces, whether of hardwood, ceramic tile, or marble. The center of the floor should be covered with an acoustically absorbent surface, such as a carpet or rug. The idea here is to have sound absorption in the central part of the floor to cut down on reflections, while keeping the edges of the room more "live" for resonance. If the best-sounding location for your piano is not far enough out into the room for the instrument to be placed on the carpet or rug, place under the piano a separate area rug large enough to cover the piano's entire footprint. The bottom of a grand piano's soundboard produces a great deal of sound that a hard floor will reflect, thus making the sound harsh and brittle—unless something is there to help absorb that energy. If you don't mind how it looks, you can store piles or boxes of music or recordings on the floor directly under the piano, which will provide absorption and diffusion. In very "live" rooms, a thick fabric cover (similar to a full piano cover) can be suspended under the instrument's soundboard. This is especially useful in practice rooms, where clarity is more important than generating a big sound.
Vertical pianos, normally placed against or near walls, don't interact with hard floor surfaces as intimately as do grands. However, if your vertical is in the middle of a very "live" space, such as a dance studio or theater rehearsal room, it can benefit from some sort of floor covering under it that extends a few feet out from the piano on all sides. If a vertical's sound is still too resonant or bright, whether the piano is up against a wall or out in the middle of the room, you can eliminate some of this by hanging a heavy fabric cover or blanket over the back of the instrument. Not very stylish, but it works.
Some high-end piano dealers will give you time to audition an instrument in your home or studio before you make a final commitment to purchase. I strongly recommend taking advantage of any such offer—the room in which you place your piano is as important as the instrument itself in determining the ultimate sound.
Lewis Lipnick is the principal contrabassoonist of the National Symphony Orchestra in Washington, DC, and an internationally-acclaimed soloist and teacher. His consulting firm, LipnickDesign, specializes in designing high-resolution audio and video systems, recording studios, and home theaters; in environmental sound control; and in the acoustical design of commercial and residential spaces. Visit his website at www.lipnickdesign.com.