The 260mW @ 300ohm numbers are directly from the Schiit spec sheet. At 32ohms the magni outputs 1.2W per channel. Yes. 1.2 watts. sounds like a lot but definitely what Schiit lists on their spec sheets.
I know the vali sounds wonderful, but it is a hybrid and still has tubes. I will be working on the same table/desk where the amp will be located and my experience is that the tube amps can tend to color the music. ( some more than others of course. I rather like the sound of ALO tube amps.specifically the Pan-AM) I really want to avoid the ting sounds you get when the amp picks up vibrations. When I am sketching new designs or working on some models at my desk I can really get my desk moving. I don't even keep uncapped beverages on my desk, tumblers are ok. lol.
THE color is teh idea, the DT 990 is a very bright headphone with a recessed mid range, the Vali is a very warm tube amp with a slighty forward midrange. The idea is to take the DT 990 and the Schiit Vali,
Additionally, the noise of the Tube is only heard on low impedance headphones, the 250 ohm DT 990 has enough resistance to filter out the noise. So trust me, the combination will do great, and the noise of the Vali isn't something you hear on a High impedance headphone! If your REALLY worried, get the 600 ohm dt 990, but it's about $60 more than the $150 990 pro 250 ohm