I did very careful and meticulous hearing test on the objective 2 and it absolutely does not alter the sound in any way. What happened to you most likely is that you played the music at a higher dB volume when using the amplifier or had gain on in your system (which actually distorts the sound). What default gain does your o2 have?
If you would like please point to a specific (minutes and seconds) part of the song and a specific thing (e.g. an instrument) that you heard with the amp that you could not hear without the amp. I would be happy to listen to that song on Spotify 320k to confirm. However if all you have is vague comments about it being fuller or soundstage then you are either lying to yourself to justify the purchase or you simply played it at a higher dBm level.
By the way, I noticed that nobody commented on the scientifically probable fact that most people who can afford to buy expensive anps no longer have the ability to hear subtle sound anymore due to their age. I've always found it hilarious when a 40+ or even 60! year old wants to teach a 20 year old about audiophile stuff when in any objective test on a simple system the 20 year old's hearing would blow the other's away.
Truth and science people. Spend your money on headphones not amps.
41 year old here and my hearing (confirmed by an audiologist last week, not by some internet test) still extends to 17 kHz. Have been listening to classical since age 15 on headphones but fortunately been wise with the volume and had the good sense to walk out of clubs where they jack up the dB level to insane heights. But I'll stipulate to being in the minority. Nevertheless, enjoying music and audiophile experience can't simply be reduced to the ability to hear high frequencies. It's also a matter of getting to know a genre, becoming familiar with the harmonics of a sound etc etc. A good friend of mine is a conductor, he's 63 now and I'm sure his hearing isn't like mine anymore but he's forgotten more about music and musical appreciation than I'll ever know.
I have the O2 on my desktop. It's good for what it is, terrific value and virtually distortion free IF and only if you use it with headphones of reasonable sensitivity and at moderate volume. Once you add extra gain, use inefficient headphones or even at moderate volumes, listen to high dynamic range music it craps out quickly. It has a terrible slew rate (ability to very quickly deal with sudden power demand shifts) and that shows in transient response, muddling of the tone and distortion. Don't believe me? Just put on eg. Stravinsky's 'Rite of spring', the 'infernal dance' movement, boost the volume, listen to the strings and the trumpet notes. Or to some of Copland's dynamic piano playing, piano notes are a dead giveaway as they tend to lose timbre when the amp can't follow. It's very noticeable on headphones like an HD-800 and certainly if you compare it to a higher quality amplifier. Not bashing the O2 but it comes at a cheap price and has its limitations (which is not the same as saying that expensive=always better).
There's a reason why in audio it's about source quality first, then headphone or speaker quality, then come the amp and the dac and finally all the rest. Which is why I have a ton of diskspace taken up by studio master flac-files (if you think a 320 mp3 file is largish, take a look at those).
As to amplifying an amp (a later post in the thread): if you listen to speakers a pre-amp can make a lot of sense, depending on your setup. Not all amplifiers are about providing gain, BTW, take a look at the FirstWatt F4 for instance, that's essentially a current buffer, it boosts the current of the pre-amp's signal without adding gain (technically it actually has -.5 dB gain) and is essentially a transparent window to what comes out of the pre-amplifier. But generally I agree that the shorter the chain, the better. Once the 0 and 1's of a digital input have been transformed into an analog signal, whatever you do can at best maintain the quality, never improve it.
Anyway, garbage in=garbage out on a GOOD system. What you need to do is pick the elements in the chain so that quality in doesn't become garbage out. Most of what I mentioned about amps will only come into play if your source is good and if you listen to good quality headphones that are at least in principle able to provide a correct sound reproduction. Remember the weakest link in the chain thing.