You are correct, the headroom is about 20dB when running in overtone, that is why I chose the 20dB amplifier, but only when you remove (or reducing with at least 15 dB) the fundamental, and that is exactly what you're doing when using this measurement approach.
When you are not filtering out the fundamental and use a 20dB amplifier the SA612 will be driven into non-linear and the overtone response will deviate from what is should be.
