Xonar DG/DGX would be a decent buy. It's cheap, but it offers Dolby Headphone and has a headphone amp, which will be of benefit to the 250 Ohm headphones. However; you have 889, which is one of the better onboard audio solutions. Technically the Xonar DG/DGX is better, and that is what most people find. However, there are some people who found it wasn't a noticeable improvement.
If you don't want to spend a lot, then a DG/DGX would still be a good choice for the features it provides. If you can spend more, I'd probably go for either a Xonar D1/DX or the OEM version of the Creative Soundblaster Z, which is around £53.
The SB Z has a headphone amp, but the Xonar D1/DX doesn't. The D1/DX is a great sound card, but a headphone amp would be beneficial to the DT990, so you may prefer to go for the SB Z. I've no experience of the Creative Z cards, but the Z has been getting decent comments from users.