Wouldn't say the DX is made solely for speakers, as it has Dolby Headphone. Without a headphone amp though, it does limit which headphones will perform well with the card without the need for an additional amp. There is a decent choice though of headphones that will work well without the need for an additional amp.
I keep forgetting about the Soundmagic's. Focusing solely on the sound quality you get for the money, they look like a very good choice. They come with a short 1.2m cable though, so the included extension will likely have to be used. Due the proprietary connector used on the cable, finding a longer cable without the need for the extension will be tricky as the brand is not so well known as Sennheiser and AKG, which probably have more choice of after market cables.
I don't know whether the build or design differs between the 150 and 200, but I've seen some negative comments about the 150 build quality, in particular the hinge cup design, which snapped on a couple. I haven't seen any similar negative comments for the 200 that I can see.
AKG K550 MkII are also easy to drive and might be worth considering. Need a good seal with these though, which is nigh on impossible if the user wears glasses.
There's also Sennheiser's new line of HD5** headphones. HD559 are within budget and 569 for £10 more. Not sure what the differences are between all 4 in that range, but assuming they are similar to the 558 and 598, they'll made decent gaming headphones that are easy to drive.
To throw another headphone into the ring, there's also Beyerdynamic DT770 80 Ohm, which will be more DX friendly than the 250 Ohm variant.
