Lets see, a battery powered Amp that runs for hours on hours with a charge, or a desktop Amp that can throw almost a watt out and run forever?

I don't know - its sort of both. It is too big to be taken anywhere, but definitely small enough to travel if you want it to. I ordered mine with a black face plate which I think is a little cooler looking.

There appears to be some "mythology" surrounding this amp, some claim it "ruins" their sound quality while others think its awesome. I think its an amp - that is, it takes a signal, amplifies it, and otherwise has no real impact on what you hear. It has no real noise I can detect (except at max gain of 6.5X with nothing playing at max volume, which is possibly my source, not the amp). Those who adore vacuum tubes might hate this discrete amp, but I hate replacing tubes every 9,000 hours, and like the idea that this thing should run forever

As far as amps go, the sound quality has been awesome, convincingly "clean", and does not seem to distort or struggle even when being pushed hard. You would have to spend at least another $50-100 bucks or so to get to the next level of power (about 1 full watt) with similar features and quality.

Mine came with small stick-on rubber feet, and a blocky 12 V DAC adapter. Having all jacks in the front is not a HUGE deal, though the presentation might be cleaner if at least the power and input jacks were in the rear. On that front, it would be nice to have digital/RCA/USB inputs as this unit probably isn't going anywhere. Whole unit has a nice solid feel, volume knob is solid feeling. Only gain and power switch come across as a little "cheap". I ordered stock 2.5X gain and 6.5X gain switch - which is about right for my HE-400s. For a more efficient headphone, as you can choose gain level, I would recommend 1X gain and perhaps 2.5-4X for less efficient cans. If you listen to many High Dynamic Range recordings (like DVD movies, for example), you may want that high gain switch - it can help reach the right volume levels with certain headphones.

JDS Labs shipped extremely fast, answered my questions, and will "customize" the unit by shipping it with gain settings and color of your choosing. You can choose your own VAC adapter, or just buy theirs, as I did.

Prodigous power from a small unit - I estimate about 700 mWs @ 50 ohms while plugged in. Otherwise, about 500 mWs reliably from about 32-100 Ohms plugged in, and about 500-400 mWs on battery. Far more power than needed for most headphones. Very low impedance output should be a great match for a very wide range of headphones. This thing specs better than amps several times its price.

That said, it also costs almost as much as very good component amps for cars. It costs as much as some very good headphones. In the world of high-end headphones, it is a fabulous value. But I would skip it if you get enough volume from current sources. The HE-400 provides a fairly good threshold perhaps. Any phone with about 95 dB or less will probably love this extra power. Those more efficient than this should probably not need it.

The JDS Labs O2 Amplifier Review

This is my first Amplifier review, but I will try my best.

Equipment Used: Modded T50RP's, Brainwavz HM5, and Dell Dimension 9200

Build Quality 5/5: The enclosure of the O2 is rock solid, each screw is tightly screwed in, I could smash holes in the wall with this small tank. The volume pot sticks out and seems like it could easily break off, but no worries it's tightly attached on. The power button and the gain button is a little too small for the front of casing and wiggles from side to side, but it's also nothing to worry about. In conclusion, the O2 is very stable.

Design 3.5/5: The volume knob is very smooth and slides with ease. In addition the engravings and the enclosure in general gives it a very classy feel. All in all the O2 feels like a 350-400$ product when it comes to looks. But, how is portability? The best way to state it's portability is; the O2 is too big to fit in your pocket, and it's too small to be considered as a desktop amp, giving you an awkward feeling when it comes to size. It does fit some of my pants, but the metal housing makes it seem very heavy. But in terms of portability when carrying it to different places, and not actually using it on the go, yes it seems perfect for just that occasion.

Sound Quality 5/5: This amp is very interesting when it comes to sound, it feels a little slow (at first), but you'll get used to it. The O2 refines and smooths down everything while still keeping neutral.

Highs: It doesn't have that extra pzazz, or sparkle, but it will suit you needs, in my opinion, I actually really like the treble that way, not too colored or bright.

Mids: The O2 takes the mids and add a hint of warmness that is very noticeable, but not in a bad way, it gives the vocals a feeling of smoothness. Also making it very lush and full, while still increasing detail, and clarity. Like I said before, this warmth also gives the rest of the spectrum a very smooth and lush feeling.

Lows: I'm no bass head, actually I find it annoying when bass seeps into the mids, but the hint of warmness slightly increases the texture, detail and amplitude of the bass, and I actually like it. In simple words; Tight, Refined, and Smooth.

Sound Stage: The O2 slightly widens the sound stage, not much, but it's noticeable. Nothing much more to say.

Driving Capabilities: It drove my Modded T50rp's with ease, with almost no effort at all, it also drove my friend's Beyerdynamic DT880 Pro's with the same ease. Also, many say it can drive any can you can throw at it.

Overall: Amazing amp for the price, for 144$ it's basically a steal, Nuff said.

Edit: You may be disappointed at first, thinking you just wasted $150 buying something that doesn't improve sound quality, but give it time, like an HD600, it won't impress at first glace, but overtime it will last, and they're both neutral. Also, in many pics it seems very large, when you receive it, you'll be surprised how small it really is, it should be the size of an average to above average hand.