Rick,
I'd suggest:
- Use an external mic. I use an Audio-Technica stereo mic and it gives me great results, with plenty of low range. Listen to some of the videos I recently posted, made with that mic. The low end of the piano really comes through well, I think. The model I use is discontinued, but here is a similar mic: Audio-Technica Stereo Mic
I've also heard very good things about a Rode stereo mic.- Move the mic back more. I think you may have a proximity problem. The ratio of the distance from the mic to the high ww's is large compared to that between the mic and low brass. That requires either a good X/Y pattern or cardioid mics probably. Omni-directional will sound too echo-laden from the longer distance.
- If you can't move it back, and maybe even if you can, try to elevate it more. That will help a little with the distance-ratio-thing mentioned above. Also consider the line of sight. The lower the mic the more the bodies of the front rows are blocking the direct sound of the back rows.