"where everything aspect of the sound is right apart from the treble: it is simply too high"
There are a number of factors at play (literally) here:
You’ve probably considered this already, but experiencing changes when speakers are moved about the room would suggest room treatments may be helpful as a first step?
I have a speaker patent, and do speaker performance upgrades. Taking a quick look at what is available out there on the crossovers there are plenty of opportunities for improving the sound which would, YES, calm down the high end and make it more transparent, detailed, and less aggressive without introducing additional (sound altering) elements to the signal path. Actually, taking things out may be a better solution to get you where you want to be.
The resistor "jumper" is convenient and "consumer friendly" but introduces more connections and a literal "switch" in the signal path. I would experiment with the supplied resistors and select a value that best works for you. Then order a good one (I use Mils from Parts Express). Remove the crossover input board and solder the resistor directly across the terminals. Even if you decide not to use a resistor, I would still jumper across this with a high quality cable. Will reduce the "edginess" of the sound to a notable degree.
The next suggestion is going to cause some immediate distress and push back from some, but I would consider bypassing the fuse. It’s been suggested, and my experience aligns with this, that amps and source devices have gotten better and more reliable over the years and the chance of blowing a fuse is almost zero. Fuses (and their holders) are terrible sound degraders. So, here we are, paying very close attention to our speaker cables, connections, etc., and present all of that to a minuscule piece of wire in a glass case called a fuse. I have noticed that there are silver or copper slugs available to do this. As with above, the fuse holder is another "switch" in the signal path (actually 2) and is made of cheap materials. I’d pull the plate and jumper around this with good speaker cable. IF this seems a reasonable proposition to you, I would do this step FIRST in that it will have a dramatic impact on the characteristics of the high end. (May not want/need) a resistor?). I know there is a risk involved here -- and you live there, I don’t. Your call. Just revealing what years of experience has determined and there seems to be a strong consensus on this with others.
Conclusion: You may get sonic performance well above your current level without adding more (sound degrading) components to the signal path.
