I have the Magni and the DT 990 Pros and find this combo to be a bit too bright, and at the same time the bass is way too bloated for most acoustic music.
I have a really good source, btw, that adds considerable energy in the upper and lower ranges, the HRT HD Streamer. And I'm using a SilverPlus USB cable, which further increases the detail and energy in the treble area.
The truth is that when I go back to my original love, the Grado SR80 headphones, the warm mid-range returns and offers a different sound, but in my opinion better for most acoustic music and vocals. Also, the bass extension is good with the Dt 990s, but the quality of the bass and the way you feel strings vibrate in the low-mid range makes me like the Grado 80 headphones better for a lot of rock, classical chamber music, acoustic blues, folk and even most jazz.
Even as good as hip-hop is with the Dt 990s and the Magni, I find the vocals to be almost too clean sounding. I'm listening to Outkast and 2pac and the vocals are so crisp they sound a bit processed. The main music that benefits from this particular set-up is EDM, which only comprises about 2% of my collection.
I may have to go and get the Valhalla 2 or some other tube amp, but I'm debating now whether to invest in a better Grado, or a darker headphone like the Senn HD 650. I do like the darker signature of my Sure 425 IEMs with this set up, but maybe I should give the Dt990s some more time with a better amp, or get the Dt880s 600 ohm? It's all a trade-off, and the cost-benefit is a huge factor for me.