This problem is at least partly solved by the amp having excess gain, or a gain switch that makes excess gain possible for unusually quiet input. In other words, attenuation by ReplayGain is more of a gain than a power issue.
I am not sure if listening to dynamically compressed music (which is what is attenuated the most by ReplayGain) at 110 dB SPL is a great idea, though. It is generally best not to exceed 85-90 dB average, and that is what gives 110 dB peaks with the most dynamic tracks. However, not all people have the same preferences, and for that reason, there is no single "one size that fits all" power requirement for any headphone. One person's "underpowered" amplifier can in fact be plenty enough for another.