What is the difference between TubeMagic D1 and TubeMagic D2

D2 is designed as a dedicated DAC. That means it has no analog inputs and  headphone amplifier output like the D1 has.  D2 is for people already have a stereo setup and using the D2 as just a digital to analog converter. D1 on the other hand has much more features and can be used in anywhere.

Is there a volume control for the TubeMagic D2?

There is no volume control on the D2. The reason is adding any kind of volume control (no matter it is digital or analog) will downgrade the audio quality in some way.  Since D2 is a dedicated DAC, we think the volume control job belongs to the Amplifier or Active Speakers.

How does the OPA627 OpAmp upgrade compare to the stock OPA2604 OpAmp?

It will give you more details and improve the clarity in the music. OPA627 is regarded as very high-end OpAmp and is worth the upgrade for any people who want absolutely best performance.

..more to come..