Audio Research pre-amps?

I doubt the ARC pre-amp is the issue.

Can you adjust the position or orientation of the speakers so tweeters do not fire as directly at listening position? That would be an easy tweak to reduce brightness.

Or if tweeters are at ear level currently, try lowering the speakers, and use shorter stands if that helps. Try them on just a few inches above teh floor even with anything underneath them to make that happen just as an easy test.

I had similar brightness issues with my Triangle Titus speakers in my wifes sunroom at ear level. I moved them to low stands just a foot or so above floor and that fixed that.

I run other speakers in other rooms off the same source system, and no brightness issues there, so my input is that room acoustics, speaker location/orientation relative to listening position is always the first and often easiest thing to address before changing anything.

The ARC LS3 is a solid state preamp. Almost any tube preamp,
(whether ARC or not) will add a touch or more of warmth and
soften up the brightness.

Its true a good tube pre-amp should be a step in the direction of more warmth and perhaps less brightness, but that alone is not a guarantee.

I use an ARC sp16 in front of the Triangle Titus speakers that I have had brightness issues with those speakers (not others off same rig) in some rooms/cases even with the tube pre-amp.

The sp16 is quite detailed and only the slightest bit warm to the point of making it hard to distinguish as a tube device compared to other good SS gear.

Other tube pre-amps likely tend towards a warmer tube like sound, but not the ARC sp16. ARC in general is known more for factors other than "warmth" I would say.

I am not a fan of the unnatural coloration too much warmth adds to certain recordings in certain genres in particular.

The best SS and tube gear tend to sound similar IMHO and not overtly warm, nor bright either, with the right setup to go along with it. ARC gear in general (VAC also) seems to reside firmly in that camp, especially in comparison to some other tube gear I have heard.

I will second Mapman's suggestion. I also have an ARC SP-16 which I use with a pair of Quicksilver Midmonos to drive Monitor audio RS-6 speakers.

I agree the SP-16 does not sound overly warm but does take the edge off the highs.