Thanks for providing these very necessary measurements.
Neither you nor HugeMan have specified which range scale you are using for these measurements. As long as the range scale is not changed, then I would suspect that calibration of that range scale would not be a factor. On the other hand, the probe compensation adjustments for that channel could have very dramatic affect.
U5 pin “Y”, is that what you are measuring? The pin numbering seems to be inconsistent on the schematic. It appears that U5 “Y” is the input to the op-amp U-7, and U5 “X” is the output of the op-amp U-7. If so, then U5 “X” will be the low impedence signal into the ADC, and that is where the measurement should take place.
I am still awaiting my replacement Quad. Maybe you could upgrade to the latest firmware, conduct the new probe compensation procedures provided by HugeMan, and then repeat your test to see if the results are different. Hopefully the probe compensation procedure will help to flatten this bandwidth. It may be robbing Peter (2-3Mhz) to pay Paul (15Mhz). If this is true then the bandwidth will probably end up being more than 15Mhz.
Thanks for sharing this info.