I recently conducted head-to-head USB comparisons with the KBL Hyperprism, Sablon Evo (still the best "bang for the buck" USB cable in my opinion and experience), Stealth Audio Select-T and the QSA Gamma Infinity.
I swapped USB cables back and forth over about 3 weeks of testing, taking notes along with way, while listening to my go to audio test tracks, in both my headphone system and my speaker system, although the vast majority of the time, I used my speaker system for testing.
I believe KBL Hyperprism is second from the top, KBL Extrema being the top digital cable in the KBL line up.
Prior to my testing, Stealth Audio Select-T was my favorite USB of all time.
After my head-to-head testing, my favorites, in order:
1. QSA Gamma Infinity
2. KBL Hyperprism
3. Stealth Audio Select-T
4. Sablon Evo
Compared to Sablon and Stealth, KBL Hyperprism had a more spacious sound, improved clarity/detail, and I thought tonality was more realistic with the Hyperprism, but QSA had a "presence" or a "the performers are in the room" aspect that ultimately bettered the others.
To put things in context, I thought the Hyperprism was a big enough improvement over Stealth and Sablon that would have caused me to purchase the Hyperprism, and if I didn't have the QSA for comparison, I would have bought the Hyperprism.
I think KBL Hyperprism vs QSA Gamma Infinity is not quite a fair fight as they are in a bit different pricing categories. KBL Extrema vs QSA Gamma Infinity would be more appropriate as they are more similar in cost and Extrema is the top of the line for KBL, while Gamma Infinity is by default the top of the line as I believe it is the only QSA USB cable.
I ended up buying QSA, one for my headphone system, and one for my speaker system.
If you want to stay below the cost of the QSA or KBL Extrema, then, you really need to hear the KBL Hyperprism. It is exceptional.
I know KBL is somewhat unknown here in the USA, but it is definitely a brand to put on your radar.
KBL Extrema vs QSA would be an interesting comparison for me... perhaps it could come down to synergy and subjective preferences, or perhaps one would better then other, but, that is simply a guess on my part as I haven't heard Extrema.
Ultimately, you need to hear each of these cables in your own system and come to your own conclusions.