[Anthony]

Here's the same horizontal versus vertical with the amp calibration in the loop.

Not much change, although there is a slight correction for the amp that seems to roll off about 1dB at 20k (but that does not explain the rest of the rolloff completely).

Horizontal is Purple and vertical is Orange.

For those of you who are so inclined, I like the voltage divider setup. It allows the calibration to occur after the amplifier so your reference measurement includes amp response. This way, you've eliminated all but the mic, speaker, and room (sadly those are the biggest contributors, though). Do NOT use the amp output as calibration unless you know what you're doing, though. You can easily fry your soundcard. It took me a lot of research to get the input impedance of the card, and I measured the output of the amp at different setting and sized the jig/divider accordingly.