Grands usually sound better when they project at an angle down the long axis of a room. This also prevents the formation of standing waves - when sound being projected collides with sound being reflected.
There are two ways to approach reducing loudness, voicing the piano and voicing the room. Voicing the piano usually means voicing the hammers. Voicing is its own skill; not all techs have it. If you like your piano's tone, I'd suggest reducing volume by voicing the piano only as a last resort.
For voicing the room, the operative words are soft and irregular. So area rugs, wall hangings, cloth furniture, bookcases with books in them. What you're trying to do is to break up and absorb sound.
Devices such as acoustic caster cups, acoustic foam, and bass traps might be thought of as including elements of piano and room voicing. Supply sells Piattino cups here.
Acoustic foam, such as that sold by Edwards String Covers
in their Sound Reduction Kits, will not change your piano's tone if you have it fitted between the beams, under the soundboard. The foam could be obtained and cut by your tech. Some foam attenuates the treble more than the bass. I don't know if this is true for the foam sold by Edwards. If the bass register needs more attenuation, bass traps, such as those by Auralex Acoustics, will do that. The traps are usually most effective when mounted in corners. (Foam and traps are both used by recording studios. You or your tech should be able to locate them in your area.)
Your situation, including your aesthetic preferences, will determine which options are best for you.
If your piano should initially sound unbearably loud, just get yourself a set of musicians ear plugs, so that you can continue to play while you're deciding which options you want to pursue. Hearos plugs have been favorably reviewed, as have the much more expensive ones from Etymotic Research.