From the angle it looks like you might have a model DR-2d. I'm not sure if the on-board mics are XY configuration or parallel A-B. If you peer through the metal mesh enclosing the mics at the front, you might be able to tell visually. If the mics cross like an X or look like a V, that's XY. If that's the case, and if you can see the two mic cylinders, move the stand slowly back from the piano rim while monitoring where the mics are pointing. At some point the left one will suddenly overshoot the tail of the piano while the right one overshoots the keyboard. Too far! You'll need to move the stand back toward the piano, probably to be no more than .9 meter maximum. To better cover the soundboard, it might even have to be less than that.
But if you can perceive the two mics as being separate and distinct and positioned parallel in the recorder case, then my first recommendation to pull back 1.5 to 1.8 meters holds, as that will be superior for A-B mics in parallel.
If you can't tell from looking, hopefully your specifications booklet might help you, or if not, there might even be a tech support phone number there.
There is another possibility that you should try, even if nothing more than to rule it out. Close the piano lid, but leave the front "flap" portion open and lying back on the larger part of the lid. Place the recorder just behind the flap and centered on the lid. I've heard a few recordings done this way and the results surprisingly exceeded expectations. However, there is no point to putting the unit on the side pedestals of the music desk. That is way too close to the hammer line such that mechanical action noises will be picked up in the recording, plus the sound will be biased either to the treble or bass.
Finally, that unit has a "limiter" switch. That should be in off position, as it is meant primarily for voice, not piano.
Thanks muchly for all these tips David, it is appreciated. I could not see the mics through the mesh, but it may be a good idea to read all of the fine manual, and indeed get some tech help if needed.