Unfortunately the Vali isn't really "out in the field" so to speak for testing. If you aren't in a huge rush, I'd wait another 2 weeks or so. By that time a good number of people will have both, and maybe even a few will have the DT880 and a Vali. With that information you'll be in a better place to decide.
A little OT, but I don't think I've really heard anyone describe the DT880 as "bright". What are you plugging them into?
Oh yes the DT 880 are on the bright side. I've owned 2 Dt 880s and I still feel they are bright headphones. Granted very balanced but still a smidge bright,
Non the less I agree, wait a few weeks on getting on. Till the hype dyes down and ppl can a get chance to flesh out the amp. But it boils down to preferance. I owned a Solid State and Tube at one time, both where quality wise on about the same level. I hapapend to like my Solid State over my tube, so I went that direction. What you like will depend on your preferance, and I would to get both the Vali and Mangi at the same time. Schiit may charge you a small restocking fee to return it, but you could also resell it here on head fi Or you can start with the Vali, and if you don't like it trade it for a Magni!