I've spent time playing with Audirvana+ and Voxengo's HarmoniEQ to correct the horrors of the HD 650 with some poorly mastered tracks (see earlier in this thread and Agnes Obel's Aventine album). Beyond that, however, I can't see using EQ all of the time to correct a sound signature I don't like. You either have to "learn" to love the HD 650s or move on, IMO. As always, YMMV.
This is true but it can become expensive very quickly. I bought the AKG Q701 before my 650 and they didn't work at all well for me - with or without EQ. The 650 on the other hand gets my preferred sound 90% right. The parametric EQ in JRiver can deal with the other 10% easily. YMMV.
However, as JamieMcC points out above, that sound card really isn't doing the business.