Quite a few people have asked me how cabinet volume affects an IR. I was curious and performed a test where I captured an IR with the cabinet quiet enough to have a conversation over, an IR at my normal volume, and an IR with the cabinet at an excruciatingly loud volume that made me nervous about potential speaker damage. For the biggest chance of hearing the difference, here's a clip comparing the "Quiet" IR to the "Loud" IR switching in real time to hear the differences between the two extremes without any audio gaps. Aside from the cabinet's volume, the only other adjustment made was the mic pre's level so that both examples hit my DAW's input at the same level.
Personally, I don't hear a difference big enough to justify capturing the same mic placement at various volume levels, and the difference you may be hearing probably has more to do with how hard the mic pre is being driven rather than how loud the cabinet is. I have no bias one way or the other on how loud to run a cabinet for the most accurate capture and I hope this comparison is helpful to anyone curious about it.
