If you look at my profile, you'll see that I've got mainly mid-fi sources, so this was my first chance to have an extended listen to higher end gear. I did most of my critical listening with three headphones, modified Grado SR80s, Beyerdynamic DT880/250s, and Grado HF-2. I did try them with customs one evening, with unsatisfactory results I'll go into below. I listened to them as a combo, only, usb out with foobar2000.

I don't have a set list of tracks that I use to test gear. Generally I'll come up with a list of things that I'm currently listening to and try to make sure that a range of genres are represented. Partly I do this because I'm just lazy and don't have the patience to listen and relisten. (Not to mention that it's not my idea of fun). Partly I think it's a good idea to throw a range of different music at equipment, music I know well, music I just got, music that's well recorded, music that's crappily recorded. This provides a more reasonable test for a music lover, imo. It gives me a better idea of how equipment will affect the usual, rather than critical listening experience.


Strengths:
Detail and accuracy: On live recordings, the details that allow you to place the instruments in space are all there. On 21st century chamber music - yMusic's Beautiful Mechanical, the timbre of the instruments (strings, bassoon) is accurate. You can even hear the bow bounce on the strings. And bass-heavy tracks also do fine. I've been listening to The Roots, Undun lately and even with the slightly bass-shy Beyers, there's sufficient thump for me, with all the details intact.

One surprising thing for me is how forgiving this set up is for poor recordings. I've got a 112 bit copy of Pearl Jam's "Better Man" that I must have torrented at least ten years ago when I still did that sort of thing. It's completely listenable. But this pairing really shines with well-recorded acoustic music. I'd start a session with the intention of doing some critical listening and just get lost in the music because it sounded so good.

Weaknesses:
Weird treble sizzle/snap on some recordings and with some headphones. I heard this the most with the Beyers and the SR80s. It's hard to explain, but on some hand claps, guitar riffs, there's this added edge that I hesitate to call sibilance because it's less of a hiss than an extended snap.

It's not completely silent. To be truthful, it's about dead silent with full sized headphones. But I heard an audible hum when I plugged my UM customs in. To be honest, I don't think customs are made for desktop setups, so this probably wouldn't be an issue for most people. The hum's not noticeable once the music is turned up to a comfortable level.

One unexpected discovery, this set up really brings out the difference in the different phones. It's not like I hadn't noticed it before (why have different phones if you don't, right?) But their different strengths and weaknesses, really stand out with the Quattro.

I was one of the ten people chosen to review these iems. You can read my full review here. These are really great phones for vocal music. They're not so great for hip-hop or anything that needs a deep, tight, punchy bass.

They're light and easy to wear. I don't love the design, preferring over-the ear phones in the main, but that's a minor quibble.

If you're interested in this sound signature, this would be a good phone to try out. I'm not sure what the MSRP will be, but if they're priced like the RE0s (<$100), it's a good investment.

I wanted to upgrade my computer sound card in preparation for higher impedance phones and eventually a desktop amp. This more than does the trick and improved onboard sound more than I'd expected.
 
The con about it needing a good power source isn't an argument against it. It turned out that my computer (Vostro 420, 2009) didn't have an extra power connection for the card so I ended up upgrading my power source as well.
 

I jumped on these when Amazon had them on sale.  I had only been on Head-Fi for a month and was looking around to expand my listening horizons. I've had them for nearly a month, but hadn't reviewed them because I wanted to get past the new toy period and listen to them over an extended bit of time and with lots of different material.
 
Fit was an issue initially. They're huge and I have smallish flush ears. There was a lot of rubbing on the right pinna until I did the flip thing and put on Comply TX-500 tips.  The best fit has come with medium Sony hybrids.
 
But even before I got the fit right, I was in love. I have the RE0 and Etymotic ER6 and ER-4P. All of these are "analytic" phones, full of detail and cool. They're all great in their fashion, but the TF10s have made me jump in my chair twice.
 
The first time was listening to Janelle Monae's "Tightrope"  There's a kick drum at the beginning that just pops with these iems.
 
The second time was Saint Columbe's "Sonnerie de Sainte Germaine".  This is 17th century chamber music. The viola da gamba rips like a crosscut saw.
 
I've read about the recessed mids with these phones and it's true, they don't work well for everything. I don't think they're good for rock music. But they've been great with  John Zorn's guitar work and other mid-centric music for me.  I don't think they're great with the EQ on my Cowon iAudio 9. But they shine with the Clip+ and iH140.

Rockbox it and you've got great sound in a tiny package. Give it to an amp and it's better yet, but not so portable.
 
After looking around at some other reviews, here are a few more details.  First off, I should say that I'm a music lover, not an audiophile (yet).  Although I can detect some differences -  between amped and unamped sources, between the different headphones and IEMs I own for example - I don't yet have the depth of experience or language to give an extremely detailed review of this little player.
 
Did I say it was little? We're talking 2" x 1" here. It would be hard to get more portable. Unmodded, it's perfectly reasonable and extremely cheap.  However, if you get it, you should Rockbox it for the amount of control (database, EQ, and a lot more that I haven't fully explored) you get.
 
What makes this stand out for me is the fullness of the sound. I don't demand overwhelming bass and the Clip+ provides more than enough for me.   The midrange is forward and detailed, as are the highs.
 
For the price, I think it's hard to get a better small player.

I joined head-fi, looking around for replacement IEMs, and found people talking about headphone amps.  After some research, I decided on the PA2V2s and couldn't have made a better choice for a beginner's amp.
 
I've tried this amp with an iPod nano, an iRiver iH-140 (rockboxed), a Cowon iAudio 9, and a Clip+ (rockboxed), with ER6s, ER-4Ps, and Grado SR80s. The amp warms the sound of the etys, but not to the extent that it distorts their characteristic sound.  I noticed a larger soundstage on the Grados.
 
But the real treat, with all sources and with all headphones is the increased clarity and solidity of the sound. I was listening to Bill Evan's Trio - Live at the Village Vanguard this morning and the amount of detail (the lightest cymbal brush, ice cubes rattling in someone's glass) is ear-opening.
 
Finally, Gary was helpful and patiently endured many email questions from me as I was making up my mind. He's the best.