I agree with you on the similarities in sound of the Fubar III and MicroAmp. I have no plans to use the Fubar III again. It's in storage with the SpitFire DAC. I should sell those two now.
I don't have a D2000, but a D5000. However, I understand that the D5000 sound is quite similar to the D2000. The D5000s offer quite a different sound from the HD650's. There's a lot more base extension that can be, at times, overbearing. Again, a good amp will go a long way in tightening up that base. The D5000 sound is also a little more forward than the HD650's. I love the D5000's for my Iron Maiden and other rock music of the 80's, sessions.
The Astrodyne will improve your Micro's sound, but only modestly so. Some recordings benefit, while others don't, especially when it comes to extending and tightening the base that much further.