Tonality wise the change from the OPA2604 to the OPA267 increases the dynamics considerably with a more nimble and articulate response, a richer more textured presentation and a nice step up in terms of detail. The top end is lot more controlled and smooth with a reduction in sibilance and harsh transients. If you want the D2 to really open up then for a few bucks more this is a worthy upgrade and I would not advise buying the D2 in stock format. You will be just wasting your time. There are a number of other FET opamps you can try that are Pin 4 and 8 carry the +/- 15VDC suitable for the D2 but if you have the OPA267 this is likely to be plenty for pleasurable listening.
Of course all this talk about the SS side of things, lest we not forget the D2 packs a pre-amp tube buffer unblanced RCA output also which gives a bit of choice in terms of amp matching and sound presentation. This particular one comes armed with an upgraded GE5670W tube which adds a nice touch of analog warmth to the pre-out stage and those looking for a slightly more liquid and engaging sound then this might be the output for them, especially those using a phono stage and tube amping.
For me the tube pre-out gives a slightly more natural tonality that I prefer to the SS output but there will be fans of both. Personally I enjoyed matching the tube pre-out buffer with a Bada PH30 SS amp for a nice mix of tonality and power without breaking the bank. The D2 also matched well with my older amps now in circulation with forum members such as the EF5 and the Burson HA-160, with either the richer more analog tube sound or the more dynamic and full bass response on the SS side. Its really up to you which you are going to end up choosing.
The D2 is a budget setup but what I really like about the D2 is the ability to tweak and mod to take it way beyond its initial presentation. Out of the box its a solid DAC and a welcome upgrade for little real outlay but with the OPA and tube upgrade the D2 is hard to beat as a budget DAC contender for those looking to make a system without breaking the bank.
The tube and SS unblanced out options give a very tasty choice of warmth and dynamics dpending on your own personal preferences and whilst it is perplexing why there is no sampling option up to 192 leaving it slightly behind some other DAC competitors I would still opt for this due that pleasing tube pre-out tonality.
Of course in time you might upgrade above and beyond and certainly there are some DAC’s out there such as the Just Audio uDAC-2496 that I think are masterly in what they can offer and for others the loss of the headphone output in the D1 and analog inputs might seem like too much of a loss but for those with existing good neutral amps and need a compelling little DAC the D2 for me is top notch.