I get the feeling that at +12dB, the op amp might be pushed harder depending on the input signal (the O2 ran into this same problem), and that's not to mention we both know 2Vrms is not quite enough.
If it was, then the MacBook Pro should also be able to push the 600 Ohm Beyers since it can also output around 2Vrms.
There is something else missing in the equation here.
It's not as clear cut as like, say... you put in a 90dB signal and then the amplifier will just somehow "magically" amp it up to 90dB on the headphone.
Because that's not how an amplifier works.
An amplifier takes an original signal and "amplify" the signal by multiplying the signal and output the product.
So whatever is put in... will come out at a fixed amount higher. There is just no way around that.
I'm not sure if that applies to all amplifiers, but it sure does apply to the cmoy and all of its derivatives. The volume knob, in essence, just reduces the input signal so that the output will be able to reach whatever dB you hear. But it's still essentially... whatever comes in, goes out at a fixed amount higher.
So a 90dB input signal can go out at like, say... 60dB. Also in the process, since the volume knob "attenuates" the signal, you're essentially losing information from the input, causing dynamic range to shrink. Thanksfully, that only happens at high gain.
The other consideration has to be paid to how much voltage/current the amplifier can accommodate. There are two ways an output signal can be seen by the headphone: either as a current (when your headphone's impedance is low), or as a voltage (when your headphone's impedance is high). In both cases, there is a hard limit imposed on how much voltage/current an amplifier can put out. Any more than that and a component may be fried, or a fuse would be blown. Higher voltage/current leads to more wasted power, so that power needs to be dissipated as heat elsewhere.
That's not to mention no op amp or amplifying circuit is truly so perfect that they can amplify any signal and return the exact same thing... with every load. In the real world, these things have physical and mechanical constraints that prevent them from being able to do their job perfectly under extreme conditions. Unfortunately, the DT880 600 Ohm does fall under this extreme condition.
There's a bunch of things that goes into this, so... no, it's not really so clear that you can say "oh, well, it's 90dB, so that's already causing hearing loss, so it should be fine!"
That is... assuming you can actually get a quality 90dB signal out of the amplifier to begin with.
If it's too hard, just... try to imagine this:
You can probably throw stone to 1 foot mark. That's easy.
Can you throw a stone 100 feet? Hmm... not quite so trivial anymore.
Can you throw 100 stones at 1 foot? Hmm... also not quite so easy anymore, is it?
But that's how an amplifier works in a nutshell. Extreme loads force it to either throw one stone really far, or more stones really close.