It all sounds like hyperbole and expectation bias. People dont compare directly with unbalanced. They get their headphones balanced and because they're expecting improvements, thats what they hear. I only trust those who compare balanced and single ended DIRECTLY.
I have heard plenty of people who didn't hear a difference. If it's that subtle (and I have no doubt improvements would be slight), not worth it to me.
Well I did an A/B comparison of the HD650 on both the Mjolnir and the E09K... both fed from Gungnir, and there was definitely a significant difference. One might argue that it is the difference in amp quality... but being fully balanced is one of the quality things about Mjolnir (and other balanced setups) so the argument that Mjolnir is a better amp includes the fact that it is balanced.