I don't have the Prime, but I own K701 and T50RP MK3 and these headphones could be easily driven by the Objective2 headamp with a gain between 2.5-3.5X, depending on the dynamic of the songs listening to. Comparing these headphones with Prime, seems that:
-
AKG K701 needs
0.320 Vrms to get a SPL of
90dB
-
FOSTEX T50RP MK3 will need
0.206 Vrms to get to
90dB
-
Alpha Prime needs
0.174 Vrms for getting to
90dB
Actually, Objective2 can push up to 7.3 Vrms on high impedance loads (>150 ohms) and about
6 Vrms on
50 ohms headphones which means about
0.7W per channel. If I'm not mistaking (Tyll's measurements and also
http://www.digizoid.com/power.php helped me here too), for the Alpha Prime that means peaks getting to about
120dB of loudness (please be aware of
http://www.sengpielaudio.com/TableOfSoundPressureLevels.htm), quite fair I'd say if using normal-to-high compressed music (blues, jazz, classics and similar) but kinda loud is using highly compressed music (feel free to check
https://en.wikipedia.org/wiki/Loudness_war). AFAIK the average SPL on normal music listening should be some where around 85dB, but actual peaks could get up to 110-120dB, depending on the dynamic of the recorded music.
Hence Alpha Prime is the easiest headset to drive from all of the above, I would really not worry about pairing it with the O2, especially that my FOSTEX T50RP MK3 is using similar drivers with the Alpha Prime and I have no issues with my FOSTEX headphones pairing with the O2.
I'm also able to pair the O2 with
Beyerdinamic DT880 600-ohms version (about 85mW@7.3V RMS which is >110dB) and unless using low-dB recorded music I'm having no issues listening to these Beyers on a good level, though their maximum power is available to about 8V RMS where they could get about 100mW of power.
There are probably amps out there able to pair even better with planar headphones, especially in respect with output intensity (mA), as we all know that planars could make a better pair with valve or transistors output stage buffers.