There’s a thread on the forum about loudness levels:
You’ll see posts about the dangers of listening at such-and-such a loudness. The numbers referenced are in decibels of sound pressure level or dB SPL for short. You iPhone likely has the same upper limit of 1.2 Volts output as mine does. So in theory you could literally deafen yourself in a few minutes by listening to the Listen Pro at the max volume setting of the iPhone. But the iPhone may have some sort of protection circuit to prevent that. Dunno.
But the real point is that your phone will not have to struggle in the slightest to output a loud enough signal for the Listen Pro. Others will have different opinions, but a sane listening level averages around 70 to 80 dB SPL. You want the headphone to spec at something like 100 dB SPL so it isn’t struggling at the loud points in a track. So 122 dB SPL means no worries at all.
The reason I got all picayune on poor M0N about volts vs milliwatts is that manufacturers post their “sensitivity” specs for headphones in both ways and the reported number can have a very different practical equivalence in loudness depending on what the units are. So a 122 dB/mW at the Listen Pro’s 32 Ohms of impedance would translate into an awesome 137 dB output per 1 volt.
When you resume headphone and amp shopping this V vs mW distinction is something you’ll have to pay close attention to, so may as well start getting your hands dirty under this hood even now.
(OK, now to post this and find out what I got wrong, grin.)