I received a production HA-2 unit yesterday.
I can still hear some background hiss on low gain (like faint pink noise) with a MEElectronics M9 (Comply foam tips), but it's not really noticeable when music is being played. The HA-2 has an auto-mute thingamabob where if the device is inactive for about 45 seconds when using it as a DAC, the amp section gets muted. When that happens, you can tell with sensitive earphones that the background hiss disappears completely until you actively use the HA-2 again, in which it takes about a second to re-activate. Again, I guess your mileage may vary with the ability to hear the hiss since some people say they can hear it (such as myself), but more people seem to not be able to hear it (handful of people here, my friend, other beta testers).
Sound-wise, it sounds as good as the last beta unit! Really, a terrific pairing with the portable setups I've tried whether it's with my iPhone (no Apple CCK required!), or by itself with my laptop. As I've stated before, the HA-2 sounds similar to the Light Harmonic Geek Out, so if you're looking for a more versatile unit that sounds great, the HA-2 should be right in the ballpark. Just like the Geek Out, the HA-2 can play PCM audio up to 32-bit 384 kHz, but unlike the Geek Out, the HA-2 can play DSD audio up to DSD256 (Geek Out only goes up to DSD128 despite using the same ESS ES9018-K2M DAC chip). The Geek Out does sound a bit smoother in the treble area on the other hand. The HA-2 has a stereotypical Sabre glare that I think people call it, but it has some extra oomph in the low-end that the Geek Out 1000 doesn't seem to have and it provides some nice warmth.
Low gain is meant to be used with most sensitive portable headphones and in-ear earphones in conjunction with the digital volume control, high gain for harder to drive ones. Low gain still leaves plenty of wiggle room with the volume knob when using the AKG K 701 (and by plenty I mean I'm on volume level 1-2 on the volume knob out of 5, low gain, maximum digital volume) and I personally don't see myself using high gain unless I plan to use my 669 Ω AKG K240 Monitor headphones.
The bass-boost is pretty handy to switch on when you want the extra bass kick and is especially useful for noisier environments where the bass can get drowned out quickly.
Its versatility really makes the HA-2 special I think. It works with Android devices that support USB On-The-Go, iOS 7+ devices without an Apple Camera Connection Kit, iPod Classic (maybe?), any media player that has a line-out feature (or maybe you want to double-amp if you don't mind), and PCs (drivers are required in Windows; OS X doesn't really support DSD256 yet). Combined with its great sound, beautiful aesthetics, and included accessories (two silicone bands, one Apple Lightning-USB A cable, one microUSB OTG-microUSB cable, one 90˚-90˚ minijack, VOOC-compatible USB wall adaptor, and VOOC-compatible USB A-microUSB cable), you really can't go wrong with the purchase. To be honest, I'm really surprised OPPO Digital is pricing this at $299 considering all it has and does. Craziness.
^ I've never seen green USB connectors ever XD
I am also interested in the bass boost in the two. Thanks! 
