I think there's some misunderstanding for what the Essence STX is rated for, or what the 10 number comes from. Actually, who cares what it's rated for? It has some kind of electronics that can be used to power headphones—let's look at how that does.
By all indications, the Essence STX has some kind of competent TPA6120-based headphone amplifier circuit (also used in FiiO E9 and some others, but the STX has lower noise I think), so that can power pretty much any typical headphone very loud with minimal distortion and problems, except for one thing. It has around 10 ohms output impedance. Effectively, you can think of it as there being 10 ohms between its output and any headphones you connect to it, which is a bad thing for some headphones and is generally more of a problem the lower the impedance of the headphones (imagine: 10 ohms is significant compared to 30 ohms but not really so significant when compared to 600 ohms).
So there are some headphones where the O2 should be a little better than the STX, in a non-trivial way. For these, the O2 should probably sound more like the E17 does, since the O2 and E17 have lower output impedance. For others (most everything? especially among full-size headphones), it should be pretty much the same, or at most just marginally different.
For those headphones, I'd consider just using the E17. That said, those headphones have low impedance but mostly constant impedance across frequency, so it probably doesn't matter much. If you end up with some high-impedance, lower-sensitivity headphones that are too quiet for the E17, run them from the Essence STX. It should do a great job with those.
Given what you have, don't bother with the O2 unless you just want a shiny new box.
