I'm not a balanced is better believer. I believe that each technology has its pros and cons and it's all up to the implementation, how the circuit design makes use of the advantages and overcomes the inherent problems.
I personally find (lots of others agree with me too) that the balanced output in AK players yields little or no audible difference from the single ended output. The difference between the two outputs is marginal at best.
In the case of the Onkyo, however, I find the difference in the single ended, balanced , and ACG to be very different. Single ended is to my ears a clear, and large step down from the balanced and ACG output, with ACG my Favourite.
Do note, however, that this is not something that I attribute to the balanced or dual ground technology being significantly better. In my opinion, it's all about the implementation. Certain circuits are optimised for a balanced output, and would sound best balanced. Certain circuits are optimised for the single ended output.
Take schiit for example, their top shelf stuff are designed for balanced use. Even the schiit guys don't mince their words when they said that the SE is just a throw in for people who want it. They did say, that if you're going SE purely, you're better off with other amps that cost less.
I think in the case of the Onkyo, it was clearly designed with ACG and Balanced in mind, and thus, the Single ended output is probably just a toss in for convenience. That's my take at least