This is the first I’ve heard of the two outputs having different tone/purpose. I’m a little skeptical.
Didn't watch the vids, but I agree, the outputs themselves don't have different frequency response. He may be using them for different purposes though.
Allow me to clarify...the outputs themselves can be EQ'd within FM9-Edit within SetUp (next to the Save button) either graphically or parametrically.
What
@WKSmith had suggested was to utilize output 1 for headphones, the 2nd for FRFR and the 3rd for FOH. The FOH output was strictly left flat response, so the FOH guy could tailor the curve as necessary for the FOH PA Speakers.
The first output by default is copied to the headphones output, so any EQing of the first output should reflect how you'd like your headphones to sound. You can selectively choose either output 2 or 3 for either FRFR or FOH, and EQ as needed.
Please observe the following examples of how each output is EQ'd. The first is for my Grado entry level headphones, the 2nd and 3rd for my QSC CP8 FRFRs, and FOH.
Since I've not yet gigged with my FM9, I've not yet needed to tweak any EQ in outputs 2 and 3. One of the benefits of owning QSC CP8s is that the FRFRs have a contour switch on the back which can select from default, to default external sub, to dance, to dance sub, to floor monitor, to speech. The contour feature makes it much easier to EQ my CP8s at the speaker level rather than within FM9-Edit.
Regards
@henryrobinett, he might simply tweak either his FRFR or FOH EQ to reflect a guitar cab's EQ response curve (the hard way) or work within the cab block and within preamp, use the filter to cut lows and highs at 80 and 6500 Hz. Because FRFRs are designed to reproduce the full range of frequency response (20 - 20K), and knowing that guitar cabs have tighter limits (again, about 80 - 6500 Hz), this can be adjusted in several places (cab block, filter block, setup output EQ).
To be clear, my FRFR's are located up on speaker stands, not on the floor. EQing is different for both locations, and placing FRFRs on the floor introduces excess bass "woof." Raising FRFRs to ear level helps clarify and reduce bass woof.
Best wishes.