As @dekay suggested, damping the piano strings (two per keyboard note---that's a lotta strings!) is the first order of business. They ring like crazy when provoked, much more than an acoustic guitar or drumset. A folded furniture moving blanket will work great.
Second should be replacing the Venetian blinds with either diffusion or absorption. Ideally, you want the left and right walls to be as alike as possible, so whatever you do to one side, do to the other. "Sweeten" the highs with Townshend Pods? Before acoustically treating the room?!
Second should be replacing the Venetian blinds with either diffusion or absorption. Ideally, you want the left and right walls to be as alike as possible, so whatever you do to one side, do to the other. "Sweeten" the highs with Townshend Pods? Before acoustically treating the room?!