Magnepan LRS+ resistors on the tweeters… which?

"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.

I have had Maggies for over 25 years.  Lots of tinkering with placement and room interactions.  For me, the best way to fix what I call a "hissy top end" was to put a sound absorbing panel directly behind the speaker more toward the top of the speaker.  This will still let you enjoy the soundstaging effects but cut out the reflected top end.  I did try the fuse treatment to no audible effect.  Also different resistors with poor results.

I’m using a belles integrated amp, which makes the Maggie’s very crisp, and using audio quest type 9 but with dbs battery pack. The cable without the battery pack believe it or not made the speakers a little bit too sharp. The dbs smoothed it out. I wasn’t the only one who heard it. The wife heard it first.

Also the other alternative which is crazy stupid expensive, is to have magnepan upgrade the whole crossover to the new x version. But like I said it’s like 2 grand for that 

@cey - I agree - this is not a 'normal' scenario for these. I'd be looking at other things to tame what you're hearing down a tad, maybe speaker cable, definitely the Naim 122X - an awful cheap n nasty piece of equipment, as I found out when I bought one as it had a HT bypass feature I wanted. It was driving my Threshold S300II into my 1.7i's. Next I tried a Naim 202 as the 122x was so bad (a glutten for punishment I hear!). Much better than the 122x but still not in the same league as the Speaker/Power amp combo. Enter an ARC LS15, quickly followed by my current Ref3 - Heaven.

Anyway - dump the Naim stuff, including, and especially the NAC A5 cable.

As @waytoomuchstuff says - the input plate is GARBAGE. The binding posts are STEEL!!! as is the awful jumper where the resistor goes. The fuse and its holder should be junked, as that is what it is - junk. Not needed. It's easy to bypass too and easy to revert if you don't like it. The difference 'proper' binding posts made and the removal of the fuse & holder was astonishing. I've had various Maggies over the last 45 years, from my 1st MG1b to my current ones, and they have all benefited, enormously, from doing the above. An LRS+ should not be sounding harsh or toppy (hot). Those awful reistors should not/never be used, IMO.

From memory, I believe the LRS+ has those little feet extensions at the rear to either tilt the speaker more upright, or have it leaning back. If yours are upright with the 'bent wire foot' extended, drop them down. Leaning a Maggie back will reduce treble. If you have the 'cantilever' type stand, did your speakers not come with four small Nylon washers, which can be placed between stand and speaker on the four uppermost attachment points? If so, try them. If not, get some to try, to get the speakers to lean back a few degrees. And as @boxcarman says, put some absobent material behind the speakers to reduce reflections. Reflections exaggerate top end, so if your particular room is somewhat LIVE, then that will help enormously. Try all things, but don't use those resistors.