You've left out some important factors, chief amongst these the "housewife acceptance factor." Patrick, are you married and is your wife involved in the decor? If so, she will want for the decor to be pleasant. This is a basic consideration in the audio system design world. Here are starting points that will preserve that:
1. Buy and place a thick area rug under the piano.
2. Find a beautiful king-sized quilt or tapestry and hang it on a wall as a decorating accent.
Both of these are available in decor from classic to modern to Southwest and will reduce your high-end reflections. For everything else:
3. Build floor-to ceiling book shelves on a good portion of one wall and populate them with lots of good books. If you have the books at varying depths the shelves will serve as a diffuser to break-up hard reflections and turn them into a short-order, softer-sounding reverb, something like the diffusers in the back of my control room at work:
If you use both suggetons #2 and #3, put them at 90' to each other.
Bob