@OP, the best placement is what you propose - speakers firing down the room towards the wall where you propose to have the piano. You will still benefit from a curtain on the glass door. You will get some reflections off the piano so it it can go in another room, it would be better. Also, it will allow you to get your chair away from the wall. And ideally, some acoustic treatment behind you listening seat would be beneficial.
There's a wide variety of speakers that will work in that space so you are not necessarily restricted to standmounts - though you do want a speaker that has good control in the bass and that will work relatively close to the wall. That said, the Harbeths and Kef LS 50s are good suggestions that will definitely work well in the room.