I measured roughly 15-20 ohm on high gain, don't remember exactly but it was around that value. Quite high for IEMs, but no big deal with full sized planars since they usually have very flat impedance curve and phase. Oh, and on low gain I think it had less impedance, again I'm not 100% sure because I took measurements long time ago.
For IEMs I use the O2, it has much better specs for this kind of headphones (much less output impedance, 0 background noise, lower gains, less output power). For full sized headphones, the LD I+ has more power and a warmer sound (after tube & opamp rolling) which I tend to enjoy more.