I have a Schiit Valhalla in pristine condition, only had it for a month so the tubes are basically brand new, and Beyerdynamic DT990 which are only a few months old.
I was hoping to sell as a duo for 500$.
It really depends how long you're willing to wait for them to sell, but if you're trying to move them quickly I don't think you'll get $500.
The Valhalla 2 was just released, so even though yours is new, it's no longer the 'current model'. Anyone can go buy the new one for $349, so you'd probably have to knock off at least $50 to make it compelling, maybe even more.
As for the Beyers, just saying DT 990's isn't enough info. What Ohm? Pro or Premium? The 990 250 Ohm Pro's just hit an all-time low on Amazon, $146 new. It would depend which version you have, but if it's those then you'd have to price them lower than current retail to expect a fast sale.
Of course, if you're not in any hurry then you should just set the price at whatever you want and wait it out. You will probably have better luck selling them separately though. Making it a package deal will significantly limit the interest.