For the STX, DON'T use the normal gain. All that does is forcibly limit the volume output, it's not changing the impedance. Unless you find your volume moving from low to high too fast, always put the STX at high gain. That's just letting it use all of it's potential. I swear, if you've been complaining about volume WHILE keeping the STX at normal gain, you can only blame yourself.
The labelling of -300-600ohm is just that, a blanket label that doesn't actually mean anything for the STX. It just means that the STX will unlock all it's potential volume. You aren't doing any harm to headphones with that setting. It's still the same impedance, same amp, same everything, just all if it's volume unlocked. Obviously, don't put it so loud that it will blow the drivers. Just put it at your preferred volume.
It can say High gain 600ohm, there will be 24-50ohm headphones (as an example) that WILL need that setting to achieve proper volume.
PUT IT AT HIGH GAIN. DON'T LOOK AT THE OHM NUMBER. DISREGARD THAT. PROBLEM SOLVED.
Companies like Asus need to stop using ohm ratings as a reference to what kind of gain you should use. There are 600ohm headphones that are easily driven in volume, and there are 24ohm headphones that need MORE power than the 600ohm.