It depends what you're driving.
If you're using insensitive orthos, the Emotiva is a solid upgrade.
You wouldn't want to use the Emotiva with sensitive low impedance headphones or earphones. It's a speaker amp. So the gain is very high. A speaker amp like this without any resistors added to the output or without the use of a cable with resistors added shouldn't be paired with low impedance, high sensitivity headphones, it will be very hissy, high noise floor on those and outright destroy someone's IEM's if they tried it probably.
The Emotiva isn't a replacement for someone for their entire headphone collection. Most headphones shouldn't be on a speaker amp. The O2 is simply a good headphone amp, for people with normal headphones that have lower impedance, high impedance, but mostly sensitive. The Emotiva, or any speaker amp really, should really be reserved for very high impedance headphones, and low impedance headphones that are also insensitive (like most orthos). Even the high impedance headphones should really be as insensitive as possible really for a good overall match. This entire thread is about a speaker amp instead of a headphone amp, because the point is to deliver copious amounts of current to insensitive (or inefficient) headphones without having to buy a $1000 headphone amp.
So if you have some insensitive orthos or very high impedance, insensitive headphones, try the Emotiva.
If you do not, well, stick to basic headphone amps.