I hear you. That might explain why RSA (more conservatively?) claims a swing of 12 to 14V single-ended with the SR-71B (even though its 4-cell internal battery delivers up to 16.8V when fully charged).
I really have no idea what inefficiencies are involved and I agree that we can't assume that single-ended swing will always equal the supply voltage, nor balanced swing twice the supply voltage, but there is at least a proportional relationship between the two, up to some finite limit set by the opamps in use as you've said, or some other limiting design factor.
Still, with iBasso claiming a swing of 32V for balanced output, it seems certain they couldn't claim a higher figure with a maximum supply voltage of 16V. And there's no way the PB2 enjoys a swing of 32V balanced when using the fully charged three-cell 12.6V internal battery (where, at best, swing in balanced mode would be 25.2V, falling to 18.0V as the battery discharges.
What I take from all of this is that your supply voltage can make a BIG difference. There are several respected posters in the PB2 threads who have said PB2 performance is at its best when used with the 16V AC adapter. That's all I need to know to want a portable (transportable) 16V supply for the PB2 (i.e. XP8000 + WI16 regulator).
I've already reaped the benefits of this approach by powering the Meier Stepdance with its maximum permissible 15V supply voltage yielding much improved dynamics and bass control vs. using an internal 9V battery. A bunch of people have joined the 15V Stepdance cult.
Convenience be gone - I want performance. :-)