I've seen a lot of people saying tube amps make the 990's sound good. I really feel it takes the heart of the 990's away. They are fatiguing at first and hurt. But if you can endure this for 2-3 days you'll be rewarded with an amazing sound and detail. The detail and soundstage is amazing! And you can't forget about the bass.
I don't find that the case personally, I've owned the DT 990 Pro twice and heard it on numerous systems. Have you heard the DT 990 on a tube amp? Tubes aren't required to make them sound good, a properly paired SS amp makes the DT 990 sound great as well. I personally found a good tube amp brings out more from them.