No offense intended. Just pointing out some alternatives since I figure you still are going to want a portable rig.
Re being at half volume: Volume controls gain, which is the same as voltage. Some headphones require a lot of voltage to perform well, and they benefit from an amp that can deliver clean high voltage. They don’t require much in the way of current though, because they’re pretty efficient.
AFCs don’t take much voltage because they’re low impedance. They need an amp that can deliver current. Because they’re not very efficient, they need a lot of current. Your THX can deliver it so I predict you’ll love the sound. Your K3 can deliver decent voltage, just not much current which means it distorts like crazy on sonic peaks. that’s probably why you’re dissatisfied
I use a FIIO Q5, which is more powerful than the K3, but it’s still a little underpowered. It works, but there is a very obvious difference between the Q5 and the LYR3
Well to be honest I don't dislike the sound of the AFC. But yes perhaps the THX 789 will improve the sound even more. My bigger dislike is the comfort. They are decent in their comfort but I definitely feel their weight compared to the Sennheiser HD6XX. I also don't like the pads on the AFC compared to the 6XX. If at least the flat part of the AFC pads which touch my head would be of the same or similar material of the Sennheiser 6 series then it would be much better for comfort. Also seeing as these are closed back I certainly feel more closed in compared to the Sennheisers. I'm definitely not used to that but perhaps in time that will change. Also the stock cable is too long so I'd have to get another one and perhaps get a balanced cable.