That's probably true. The issue with multi-BA drivers is the variation of impedance across the frequency range. In theory a single BA should present a more consistent load at all frequencies and therefore not have significant fluctuations to the power delivered (and therefore the volume at each frequency).
I went looking for impedance vs frequency graphs for the FAD Heaven IV but couldn't find any.
In theory, damping (the ability to precisely control the movement of the driver) should still be an issue with such mismatched impedance, but maybe the Heaven IV is well damped mechanically and doesn't rely on electrical damping.
I'd be very interested to hear some opinions from people with the AK100 and other high end devices with low OI because the Heavens are an interesting prospect at their price and reported performance
Got the AK-FAD Heaven IV special edition earlier but can't say I m impressed with it on AK100 (stock). For single BA I much prefer the pairing with Er4s.