That's a dynamics description, AFX3 has way better power amp modelling than AFX2, and new richer speaker impedance interaction. At low volumes like you're used to playing your real amp, speaker and PA interaction is nill in a real amp, LF resonance won't be audible yet, it's basically preamp tone and speaker filtering. In AFX3, even with MV on 0.07, the speaker resonance (LF resonance) is in its own dimension, booming away as if the MV is already pushing hard, just need to turn down LF Resonance if you're going for that light-touch low-volume Mesa just-the-preamp crunch that tracks the string 1:1 as fast as you can go...with LF Resonance up at default setting the extra low end is eating into the power bandwidth of the virtual power amp as if it were turned up, effecting the preamp response in a way that's not characteristic of the amp when used at very low volume.