Pros: Neutrality, Definition, Cleanliness (Black Background), Articulation, Speed (PRAT factor), Imaging, Transparency, Power, Value (Price/Performance)
Cons: If you prefer a certain sound signature from an amp, it has no obvious character of its own, therefore it depends on our source and headphone choice
I have always loved the Grado RA1 which uses the high output NJM4556 op amp. It was one of my very first introduction to the world of headphone amping, and for years it was the mainstay of my main listening rig, both desktop and portable. The JDS O2 amplifier is using similar op amp(s) for its output stage ( and doubles it!), thus I found this JDS Labs Objective2 amp to be easily lovable right from the first listen. It does bear a resemblance in sound to the RA1, but it has more definition, cleaner sound, and more power as well.
I had owned a lot of affordable amps: the various CMoys ( I have owned most major CMoy version:JDS, Zigis, Penguin Coffee Amp), 3 Channel PIMETA, Schiit Magni, RSA Shadow, XCan V2, GSP Slee Solo mk2, RA1, PA2V2, Creek OBH11 amp, and Fiio E12, but the time had come to downsize the number of my amps. I sold the others (I need the money to get an iPhone 5S, which is unsubsidized in my country's market) and kept the O2 as my one and only one amp after months of evaluating them side by side. It came down to these three: Schiit Magni, RSA Shadow, and JDS Labs Objective2, and then I picked the O2 out of those three. I haven't regretted my decision ever since; I very much enjoy listening to Smooth Jazzes, the likes of Lee Ritenour, Earl Klugh, Fourplay, David Benoit, Dave Grusin, Incognito, etc. with this amplifier. It's paired with the following: AKG K702, Audio Technica ATH CKS1000, Philips Fidelio S2, or VSonic GR07 mk2, and also with the ODAC, Rockboxed Sansa ClipZip or the iPod with DIY iMod as the source. I love how the amp effortlessly disappear allowing me to enjoy just the music.