As to the question on the Modi / Magni combination, I really hate when threads get interjected with recommendations from people who tend to usually push the suggestion of what they have out there because it is something they like and the person asking the question may not like the sound at all.
Thus, despite my normal reluctance to do so, I thought I'd just toss out a suggestion based on the fact that I've tested quite a few combinations for my Grados (PS-500, RS1i and Magnum X build) at the current moment, plus several pair on the past.
At the moment, my FOTM (Flavor of the Millenium) is as follows: Little Dot I+ amplifier with a great set of 5654 tubes installed. For the DAC component, I'm using the HeadAmp Pico upsampling DAC. This sweet little DAC is built around the Wolfson chipset, number WM8740.
This combination just seems to do everything right for what I need to drive my Grado collection. Great bass, excellent mids and a top end that doesn't have too much sizzle.
Again, though, I'm just tossing this out there as a suggestion based on my person experienced with these devices vs. many others in the past. Enjoy!