The DSS would be your DAC. Digital to Analogue Converter.
Chicolom used an M-Stage product for the longest time (now he has an iCan for amping) with his Q701 and he was very pleased with it, I don't know this USB product you mention but it comes from a respectable brand.
I've got a pretty pronounced widow's peak, heading in the same direction as you, was it a relief to shave it all? But back on-topic, I have also read great things about the DT990, if you think the treble is a touch too much (seems to agree with Mad's observation) then you can choose to either exchange them for another pair and see if you are OK with the treble's edge, or look into other alternatives. I don't know how their track record stands for durability, but the Planar Magnetic HiFiman HE-400 may be a contender for you (still strong but perhaps less so in the treble than the DT990, slightly better in the "Fun" department if you like V-shaped freq responses), also check out the AKG K712 or K701 Anniversaries. The X1 may be more bassy than the K701 anniversaries, BUT if you read the review or go back to Mad's posts when he was experiencing the Annies he was very excited and pleased with them. I personally have Q701, with a baby-seatbelt-wrapper on the headband to alleviate the headband fatigue (feels great now), and though I wouldn't say the bass stands out as louder or more prominent than the rest of the freq range, I WOULD say these are definitely fun headphones with a bit of magic.
How are you liking the Sound Blaster Z? I'm thinking about picking one up, but I got a hint from Creative's social media team that they're working on an update to the Recon3D USB external sound card that I had before and liked.