This is my second attempt at using a portable amp (I returned my Fiio E12A for this).
 
That was a good decision. The Cayin C5 is an absolute powerhouse. It can power IEM's and earbuds (what I use it for), but also large size over-ear headphones. I am very pleased by the sound. It is slightly warm with a really expansive 3D soundstage. But by far the best feature is the bass boost; the sub-bass really slams. The amp can also charge your other devices (like your phone etc.) which is an added plus. I got the black edition which aesthetically looks great.

I used it with my Astell & Kern AK Jr. player and Rose Mojito high-fidelity earbuds.

 
The Fiio E12A Mont Blanc portable headphone amplifier is a great amp for IEM's and earbuds. The sound signature is slightly on the warm side, and presents a much fuller sound and wider soundstage than normal when using it with my AK Jr player. I particularly love the bass boost as it adds extremely clean sub-bass which doesn't bleed into mids, and is still acceptable in quantity (I believe it's +5 dB or so). The sound differences are not drastic, but they are noticeable if you A-B test the AK Jr and the E12A. I love the sensation that the sound has become fuller. For the interconnector I used the Fiio L16 3.5 mm to 3.5 mm silver interconnector.

Design-wise it looks really good, especially with the AK Jr.
 
Overall a great amp, and especially useful for IEM's and earbuds in particular (for which it is made for). I use the earbuds I have on low gain, but that's because they are all low impedance. I will test a 300 Ohm earbud soon on it and see how it holds up.

The VE Monk is a $5 earbud that produces high-end quality sound. This type of value is pretty unfathomable. 

Pros: Surprisingly good bass for $5 headphones, the foams included with them (and not sold anywhere else) are probably the most comfortable foams I've ever tried, vocals are not recessed, soundstage is mid-level
 
Cons: bass could be better quality, soundstage could be wider, could have more details
 
Sound signature: warm-ish, "sparkly" sound, very energetic; perfect for hip-hop
 
Overall ten times better than Apple Earpods because they are more comfortable with similar bass, way cheaper, and very non-fatiguing (they won't hurt your ears if you listen for hours).

This is it. I have found sonic nirvana. The Rose Mojito's - 3D printed earbuds from a small startup company in China, offering a dual dynamic driver (one driver for bass and one for treble) - which is in most high-end speakers and over-ear headphones but never in earbuds - are an audiophile's dream.
 
After listening to the Rose Mojito earbuds with the AK Jr high-resolution audio player, I have to say that it is a masterpiece of an earbud. A work of art even. I can't explain it. It's beyond the class of other earbuds. It's in a god-class of its own. You have to experience it for yourself. 
 
I am absolutely enthralled in the bass and wide soundstage. The bass is "curved" almost, and is not static, while the soundstage is so wide you sound as if you are in a large hall rather than a studio room. The combination of these factors make MP3's sound amazing, and for FLAC/WAV/AIFF (lossless) audio, leaves one breathless. And this is all without an amp, since they are low-impedance so they can be driven out of a smartphone or normal audio player like an iPod etc. Also, they offer ultra-detailing, so you can hear things in audio that you wouldn't be able to using other earbuds.

The tonality of the Mojito's is "digital" if that makes sense. Biggest surprise is how GOOD the bass sounds from a supposedly neutral sounding earbud (since the sound signature is not warm, but neutral)! The quality of the bass really makes it stand out (over the quantity), and not many earbuds have good quality bass (though plenty have lots of quantity bass).

The only problem with them is that vocals sound slightly recessed (in the background). But only slightly, and it's not a big problem considering they are 10/10 on all the other factors.

In terms of overall sound I am told they are kind of like Stax Earspeakers but in earbud format (for those into full-sized headphones). 

The Rose Mojito's are really irreplaceable, and sound like no other earbuds I've ever heard. They made my auditory cortex go into another state of mind, literally.

I have to say...I'm really enjoying the Ostry KC08 high-fidelity earbuds.
 

Pros: The reason I like them is the unique way the bass and sub-bass sounds. They sound almost like good car speakers with the bass turned all the way up. And yet the bass is not overpowering. It's just the right amount. Ostry KC08 are quality headphones for people who like bass, and perfect for certain genres like hip-hop. Soundstage is medium sized (not as thin as Shozy Cygnus but not as wide as Rose Mojito). Also, I really like how they look aesthetically; they are very unique. The cable is one of the most tangle-free cables ever (it barely tangles) and feels soft to the touch, and there are foam covers included that have a unique design allowing more treble to go through while still being full covers (as opposed to donut foam covers that expose the mid-region). Also they make instruments sound very thick and soulful, especially the Rhodes piano in jazz music.
 

Cons: The vocals are slightly recessed (though barely noticeable) and "dry" sounding, and the earbuds don't display as high detail as some other earbuds (like the Rose Mojito).