The WA6 is a transformer coupled tube amp. If you had asked that question about the WA3, I would have said to go for the 600 ohm versions for sure, since that will mate best with an OTL (output transformer less) tube amp. but with the WA6, you could use any of them, but its not the case that the 600 ohm version would be the ideal choice purely in terms of it's impedance. For the WA6, the 250 ohm version might actually be a better choice, just in terms of the impedance.
However, I do think the 600 ohm Beyers generally sound better than the 250 ohm versions. I have never compared the two on a WA6, though.
Low impendence headphones work better with wa6 but like Skylab said the 600 ohms are supposed to sound better than 250 ohms. I owned the wa6 and 600 ohms dt880 and the wa6 does not have the power to drive them and get the most out of those cans.
If you have to have the 2 I highly recommend that you get 6fd7 or 6 dr7 tubes since they have much more output .