For those interested, here are some thoughts on the O2 now that it has arrived (JDS version.)
In terms of its pairing with D5k, it's an absolutely perfect match, and sails beyond even what the AVR did in terms of detail, clarity, and transparency. It bowled me over somewhat, though the Denon AVR provided a bit more warmth. It's unbelievably precise, sounds very strong, articulate, and ultra precise. Combined with D5k it gives me an idea of what HD800 highs may sound like, for better or for worse. It's a bright amp. Very bright. Or more to the point, neutral, and combined with a bright headphone. I get a black background with the Denons, just as I hoped, and the amazing clarity, as well as the very tight bass of the Denons really get a chance to shine.
It also pairs well with the HE-400, depending on your tastes. On a purely technical level it really shows off what the HE-400s are truly capable of in terms of detail retrieval and precise transients. It raises them up a level of technicality, and also makes them considerably brighter and drier than my Lyr (hybrid) does. It somewhat turns my HE-400 into what I imagine HE-6 sounds like, and that's with pleather, not velour pads, and no silver cable. I can only imagine how it gets accented beyond there. While the sharpness and detail is amazing in terms of wow-factor, however, I think I have to conclude that I prefer the syrupy sound of the HE-400 on Lyr significantly more than on the dead accurate Objective 2. The all-too-pure sound of a completely accurate amp doesn't convey the same emotional character of listening in a large room to live music that the Lyr provides. It's interesting both ways, but it also has me less certain that I ever want a silver cable!
I can't try it with HD650 for the moment since the drivers are still dead, and I don't think I have any inclination at all to subject myself to it with K702. The double-analytical pairing would be too much for the ears to take for any purpose other than as a studio production tool to analyze faults.
Overall I will say that O2 sounds very expensive, and in a fancy case, would please a lot of people. I do wish the designer had not taken certain shortcuts both for portability and for proving the point of cost/value. The 1/8" jacks are no substitute for 1/4" jacks, but worse, they feel kind of cheap and stiff, there is no smooth connection as their is in, say, an iPod. When I plug in it feels like I'm trying to shave the jack apart. They are the sky-blue Kycon specced in the original design, are a $0.69 part that feels like the cheap plugs on the back of a PC motherboard. A $2 upgrade could buy a LOT of performance there. The Alps pot is at least a decent Alps, but it doesn't have the silky glide that the Blue Velvets have, and there is some peridoic channel imbalance on it, unlike the Blue Velvet line. Again, a few bucks could have really improved things. I won't comment on my preference for rear-RCA jacks and 1/4 jacks too much since that's outside the design, much as I'd prefer it, but the parts that are there, the user-interactive parts could have been done a bit better. The power switch while durable enough, is the type with the relatively loose cap over a loose spring and as such feels a bit like a wobbly cut-rate button. It's fair enough to say you don't need designer parts to sound good, but user interactive parts such as volume pots and jacks are a part of the overall usage experience, and I'd gladly pay $10-20 more to get nicer quality on those components. The amp design is great, but if anyone were to start producing it with higher quality appointments in terms of chassis, internal PSU's Neutrik jacks, good switches and a Blue Velvet large dial in a box, it would jump up in value considerably. Still the ultra clean sound isn't for all uses. It's a great amp to have on hand, and I prefer its sound to my Headroom Micro Amp (Desktop.) And I prefer it above everything for D5k. But for HE-400, I prefer the Lyr's warmth and smoothness, for K702 I think an analytical amp is always a bad match, and I can't try HD650, but I imagine my thoughts are same as for HE-400 there. O2 is a great SS amp, it would be an even better SS amp with some better quality user-interaction appointments, but it's no replacement for tube and hybrid amps if you like tube and hybrid effects.
This comes down to where I start to disagree with the designer of the O2 (along with most of the measurement crowd) in their crusade against tubes and coloration. For studio work, they are correct. For leisure listening, I think their viewpoint is based on a false premise. The assumption is that superior measurements are superior equipment, and I'm not sure that's true from the listening perspective. Sure, for studios, the more bit-perfect to the recording one gets, the more valuable the analysis tool. Thus the Benchmark performance. But for listening there's a schism. Some audiophiles prefer bit-perfect representation and maximum detail retrieval from the recording. Technical merit. But others use audiophile gear to try to recreate a live venue. The sound at your ears, at your seats, in a live venue will not measure perfectly with what's on the stage. Far from it. The venue has been designed in a way to color the sound. A recording is taken, in most cases, as a "stage level" recording. It expects that your room treatments and speakers will reproduce the listening environment. With headphones, we have to rely on gear to color for the listening environment since there are no room treatments in the way. I think that's where head-fi-ers get bad reputations with the measurement crowd. Measurment-perfect gear works for studios, and for speakers (assuming appropriate room design.) I think measurement perfect is in fact often undesirable for headphone gear. But if you like T1 or HD800's appeal, O2 would appeal to you as well. It's for the true detail seekers. And in that role, it does the job exquisitly. It may be bright, it may be sparkly, it may be sharp, but it has absolute control and authority over the headphones, and that can be a good thing.
Perfect for Denon. Which is why I bought it. And it offers a very different presentation of my HE-400's as well, should I ever want to feel like I went nuts and bought HE-6....