It simply depends on what you count as "listenable" volumes. Sure an iphone can power hd600's to a low level of volume which may be perfectly "listenable". The issue is power required to raise the volume is not linear, each extra decibel requires a larger increase in power than the previous one.

Now lets say you're not listening to highly compressed professionally produced music which has a tiny dynamic range, lets say its amateur produced with a massive dynamic range. Suddenly you need 10x the power to get to that same "listenable" level.

Then there is the question of what is "listenable"? If you're trying to recreate the experience of a concert you may be looking at transient peaks of 100db, that will take way more power for the hd600's than an iphone will deliver.

At the end of the day though, the question is are you happy with the volume you're getting in all scenarios? If yes, then there's no problem.