Interface calibration and Preamp distortion (THD) measurements (not room acoustics)

[rpschultz13]

Hi, new to REW. I'm using REW through a Motu M4 on a Mac.
I'm testing an acoustic guitar preamp that I've designed, it has TS input/output.

• I read (AI) that going from unbalanced to balanced I should float the RING of a TRS on the OUTPUT of the M4 into preamp, but simply use a TS cable between the output of the preamp and the INPUT of the M4.
• I calibrated it at -12dBFS, I measured 72.5 mV. This seems low, and in the generator it won't let me put in a signal more than 289 mV, which is 0 dBFS. M4 says it can output +16 dBU which is +/-3.6V. Why won't it let me output more? Not sure I need to, just wondering.
• After doing this calibration, I measured THD at 3 different preamp gain levels (top left knob), see below.

I really have no idea what I'm doing. What should I do differently or better. What else should I be looking for?

Thanks!

 

[rpschultz13]

Wow I gotta say doing bode plots on REW is much easier than my scope and AWG!