And not always.
Play an open chord, oh wow... the frequency response is very similar.
Play a palm mute... oh wow.. they're really different.
Seen that exact thing more times than I can remember.
And the problem is, you get guys who are heavily open-chord type guitarists then claiming that A capture is extremely similar to B device, and guys who are heavily palm mute type guitarists claiming the opposite.
Frequency response is a 1-dimensional - okay, 1.5-dimensional - tool when determining the accuracy of the capture of an amp as a system.
No offense required.
what a perfect summary. This is the thing I currently find with proxy. It seems fairly accurate until you start chugging. There is an exaggerated thing going on that causes certain frequencies to jut out. This is with the amp models as well. With my personal stuff, I had a few exaggerated instances around 160Hz but that could be the harmonic. It might be at 80 or simply material dependent. I think it is a lack of compression on the clone, where the amp would naturally compress more.
My overview:
Null tests by themselves are useless, especially when summarizing into a single LUFS number. The equivalent would be rating a movie from 1-10.
You're better off listening to the diff file and hearing what is different.
Frequency graphs are simple snapshots that are one-dimensional. If you can hear the differences, they may help confirm what you hear and pinpoint some frequencies.
As you have also stated previously, these tests are not taking "sag, compression, or other non-linear phenomenon going on in the circuit"
compression is a huge factor and one of the reasons I don't jive using other people's presets. Wayyyyy too much gain. I rely on heavy picking to distort and use mid gain. If people put their gain at 8 on a preset, I roll it back to 5.
Frequency graphs don't show where notes are picked hard or soft. Null tests neither. You could run multiple null tests with different input signals where dynamics are taken into account and it would give you a slightly better picture. Slightly.