@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.