I am enjoying the sound signature of the Fidelio X1 a lot more, I'm not a basshead by any means, but the HD 598 was just too light on the lower end for me. The Fidelio X1 has just the right amount to my ears, the lower end is more comparable with the Beyerdynamic T-50p I have, but the mid range is still very clear and not nearly as recessed as on the Beyers. Soundstage is also very good, been listening to them for about 9 hours now, and so far I'm incredibly happy with them.
Well, I have been travelling a bit over the past two weeks due to work but have also managed to listen to all three of my headphones in combination with the sound cards. I would say 7/10 times I am grabbing the AKG's to use with the Creative X-Fi Titanium HD. However, the Philips X1 with the Xonar ST is not a bad combo at all either. The Beyerdynamic's are so obviously outclassed by the other two that I don't even have them out of their case anymore and they are being prepared for Ebay.
I'm working on that huge extensive review and hopefully it will be up on here for everyone to get detailed impressions of all three headphones and the different combinations.