I have owned these iBasso headphone amplifiers: T2, P2, D1, D2 Boa/Viper and P3. Times and again, high gain setting has provided better dynamic range than low gain setting.
My ideal gain feature will be automatic gain control (AGC), where the amplifier adjust gain constantly based on amplitude of input signal (feedback loop is necessary). Thus regardless of different signal amplitudes from different audio input sources, output amplitude will be maintained the same and dynamic range maximized without clipping once output volume control has been set. The AGC control avoids clipping of output signal when input amplitude is too high. Setting of output volume control to match headphone characteristic is just a matter of set-and-forget.
Of course such AGC implementatio should not compromise dynamic range and sound quality at all and should not raise cost drastically. Perhaps AGC has already been implemented on chip in some opamp designs. AGC is not a new concept at all, I learned about it some years ago when I was working on read amplifier circuit of hard drive. Since air gap between magnetic head and spinning disc surface vary constantly, AGC is used to maintain level of raw analog read signal before digital decoding can be done on incoming raw bit stream (analog level of digital signal affects digital decoding to certain extent).
