You say in general panels are as flat as cones.
F'rinstance Martin Logan CLXs are +-3dB from 56 to 23000 Hz. How much flatter would you like?
They sure sound good as well. I have used panels for around 40 years now. I don't miss the last octave, yes and octave from 55Hz to 27.5Hz, although bottom B on a Steinway Model D (the lowest note in music) is 30.87Hz. For those that do, get subwoofers.