I have a Hornet, the iRiver H140 (which has a pretty hefty output on it's headphone jack) and the Ety ER4-S. Also the Senn HD650 for home use.
This is going to be cliche, but the first song I pressed heard through the iriver>hornet>ety's was A Perfect Circle's "The Package" and I heard sounds I didn't know was there. The most obvious difference to me between amped and non amped was that with an amp, is that everything has a lot more (and natural) decay. I imagine this is because the amp is souping up those super delicate echo parts of the audio and they get all the way through to your head/earphones, as opposed to the hp out of the dap which I can now see was more or less going "meh... not loud enough, won't bother!). Not really a matter of volume but sensitivity. So it's the sum of all these little extras you hear. I won't pretend it makes much of a difference for really heavy metal. It does make a difference, but definitely less so than music where the instruments and singers have room to move. I find my metal benefits most in it's more laid back sections.
The Hornet most definitely colours the sound. Keyboardy effects on Jean-Michel Jarre's "Metamorphosis" CD take on (what I find to be) a very nice edge. Phase and flange effects are so much clearer. Who knows whether it's closer to what was originally recorded or whether it's mostly the Hornet's influence, but man does it sound good. I read somewhere else that someone else used to only hear warbled effects in some songs and now they can actually not only tell it's a human voice, but what the voice is saying too
I've found exactly the same with some Machine Head tracks, which have several layers and sometimes the mix gets cluttered going straight from the hp out, but from the Hornet, things are separated a lot better.
Side note: I must say, I first listened to this setup using no EQ. The Ety's lack bass impact without EQ'ing. It's there, but it's diminuitive. With classical it's fine, accoustic stuff fine, but once I got to electronica and rock/metal, I realised I would do my ears in if I turned up to the volume I wanted to achieve the kick I like for guitars and drums. So after rockboxing my iRiver, I've not dialled out some of the higher frequencies to make up for the lack of kick (note dialled OUT, since gain shouldn't be added if u want to maintain SQ). This is purely an Ety thing, as when I listen with my Senn HD650, the Hornet and Senn's are VERY well balanced. This is most likely because the Senn's are naturally darker/laid back. With Grados or something like that, I expect they'd be brighter. I'm hopefully going to listen using some MS-1s and see.
(After fixing the bass deficiency in my Etys) .. Fast bass lines are more well defined and there is even more tone to them, I used to think the bass lines were all the same on JMJ, but now I can hear how he has applied difference effects on each track.
Anyway, if you pick a track/album you know very well, and play that first when u receive ur Hornet, you will hear a few new things straight away. And I will vouch for the Hornet changing slightly with burn in. Mine's probably seen about 70 hours (nowhere near what Ray recommends as fully burnt in) and it has definitely become a bit more fuller sound than when I first received it (which seemed slighly bright).
Add to this the fact that I'm not even using lossless or CDs (just 32-320kbps VBR lame mp3) and I'm sure you'll find it worth getting a Hornet.
With the Shure's I expect you'd hear a difference. Does the Nano have a line out? I have NFI. Don't feed the Hornet from the HP jack
I can't wait til I hit 200 hours playtime and see what it sounds like. Hope this helps.