I started out with the Micro Amp and Spitfire DAC. I really, really enjoyed them and then I upgraded to the Ultra Micro Stack.
The general sound presentation was about the same leaving the general enjoyment factor very similar. However, the jump in detail was easily heard and brought a wide smile to my face. The base presentation was also improved.
The Micro Stack is the best I've heard, so if I give it 100, then the Micro Amp and Spitfire would be about 85%.
I currently use and enjoy my Micro Amp with Astrodyne at work, but I use more modest cans, currently the Sony MDR 7506 and 7509HD. I will soon be replacing those with an Audio Technica ESW9a. At work, I rarely am listening solely to the music and hence a uber-detailed presentation isn't required, but just the wonderful sound signature of the Micro units.
To really appreciate the Ultra Micro Stack and its differences with the Micro, you'll need to pair them with decent cans.
My Ray Samuels amp, the P51 Mustang is very good only with IEM's. While it sounds pretty good with full cans, the Micro is better and the Ultra Micro, a lot better, especially where the music demands good power from the amp (broadening the sound stage, while tightening and deepening the bass).
Finally, I too have a feeling that the Ultra Micro Stack is very under-rated, especially after getting this P51 Mustang that is so highly rated.