Sorry you’re having a bad experience. Where are you located? It might be worth seeing if there’s a local head-fier in your area who can let you audition balanced cables and maybe another amp.
I have spare HD 600 cables— I bet most experience head-fiers do lol.
You may be expecting too much on the 600 or there may be something physically wrong with your cable or the k7 may just be that much better from its balanced output. The science quote on here is correct and I don’t quarrel with it but that doesn’t mean that the design choicea don’t lead to better results from the balanced out. Wave Theory on YouTube has explained this wonderfully many times where he knows-given his science background and expertise with dozens of amps by now-that balanced doesn’t actually make a scientific difference so it shouldn’t matter, but in his testing it often does in real life simply because it’s designed that way.
Whenever I have a balanced device, I always use that output because I know that it has been designed to be used that way, even though the reality is designing a single, and then output is more scientifically sound. (JDS Labs and Benchmark both feel very strongly about this and their white papers are convincing).
In the end, people end up, having repeated experience where the balance down put on the equipment sounds better, and therefore draw the incorrect, logical conclusion that the scientifically balanced must be better by design.
If you can order more powerful model, and cables from somewhere with a good return policy, I would also consider that as a way to audition equipment. At the end of the day you gotta try different things to get a baseline knowledge and experience. Otherwise, we are all just guessing in the dark, although you will find many helpful people on here.
Good luck in your journey and keep experimenting. We are all addicted to the tummy because once you find the right synergies within your budget is worth the time.