I was at the Source event this Saturday. Had the chance to compare Utopia against HD800 and HD800S with the music I familiar with. All driven by the Cavalli prototype amp at the booth. Note that I do not take price into consideration AT ALL:
Utopia vs HD800: Utopia has a great high. Just as detailed as HD800, but without the fatigue. HD800's highs feel kind of "cheap" and in-your-face. The midrange and bass on Utopia is full and engaging, compared to the hollow, emotionless midrange and midbass of HD800. The HD800, however, has a slight advantage in overall transparency, and a huge advantage in sounstage. Even though cello sounds too bright on HD800, I can tell how far away I am from the instrument better than with Utopia.
Overall, these two cans' sound signature (neutral vs bright) are too different to be compared. Each has its pros and cons. If priced equally, I'd say go audition and pick the one that meets your taste.
Utopia vs HD800S (with Kimber cable): I realize the real competition starts here the second I play music on HD800S. Going back and forth a couple times, I can confirm that the two are tuned very similarly. Utopia has slightly more bass quantity.
Both are engaging in the midrange, but instruments are more separated on HD800S. Both have detailed, extended yet controlled highs, but HD800S conveys a better sense of decay and timber. Bass are equally detail. Neither has prominent bass, but the Utopia has slight more bass quantity. The HD800S, again, takes the slight advantage of overall clarity and transparency, possibly because of having slightly less bass.
Overall, these two cans worth a toe-to-toe comparison (neutral vs neutral). Even though Utopia has a little more bass, it was for from the quantity that can make Utopia stands out as a "bassy yet good" headphones. If priced equally, with advantages in so many categories and only one disadvantage (a little too bass-light to my taste) which is neither a strength of Utopia, HD800S won.
Oh, and did I mention Utopia looks (in my hands) and feels (on my head) cheap standing next to HD800S and its sibling? The comfort and fit with Senns are simply on a different stratosphere.