Hi Dave,
Thanks for that more extended account of your trials in that hall. My second guess was right, the ambient noise being the heating system. That happened to me once right in my own living room, but during warm weather. The A/C came on, I didn't pay it any heed and proceeded to record a Bortkiewicz piece. Well... my sensitive Earthworks mics captured all of it more than I would have ever believed! The recorded sound was very good, but not excellent. So thereafter in summertime, no matter how hot outside, I always shut off the A/C before recording.
Unfortunately, you can't control the heating plant in the hall, so that will be a given during winter. I can see from the picture too how confining the stage area is, which is very limiting in regards to placement of the Tascam. The reason the chair in the first row didn't work well was because of the piano's elevation, the music overshot the height of the chair.
Putting the recorder on the piano desk stand will render a harsh sound because not only is it close-in recording, but that position is way too close to the line of hammers--meaning the recorder will pick up the percussiveness of hammers on strings, which translates to sound in the making, not finished sound. OK for jazz, but not classical.
Here's something, Dave, you could try next time: Close the lid (but not the front flap of it) and put the Tascam on top of the center of the lid just behind the open flap, such that it rests on only one layer of wood (not two). The ambient heating noise will find it harder to compete with the recorder in this placement because the piano holds the advantage. And the problem of differing elevations of piano and recorder becomes moot--they're one and the same. I've never done this myself, but some pianists have produced some really good recordings this way. You'll want to perhaps test with three increasing recording input levels to hear which works best (once you're home to listen to the outtakes). I'd give it a shot--nothing to lose.
David