Help me to tame brightness

I'd assume it has to be the speakers, and what I would do is continue to allow them to break-in. I think you may have broken in the tweeters, but not yet the mid and LF drivers. Give 'em a couple of hundred hours before starting to worry.

I think any speaker should be broken in for 300 or so hours before you consider it's sonic merits

Keep in mind also that it's entirely possible your new speakers are just a bit more revealing than the old speakers. But assuming your new speakers are not the main problem, I'd have to guess it's perhaps a little bit of everything.

However, with a bit of reseach, you might be able warm it up with a certain richness.

For example, a Rotel amplifier is a solid state known to be somewhat dark and rich in sonic character. This could be exactly what you need to balance things out a bit.

Based on your words, it also sounds as though that you have not implemented any type of AC line conditioning. If your equipment is on the bright side of life to begin with, then unconditioned AC will only add distortion and harshness to an already bright sound.

-IMO

Take a look at dakiom.com for the $57 introductory stereo system special of two "feedback stabilizers" which may immediately reduce brightness. Also, I have found that running the "de-magnetizing" CD called the Densen D'Mag with its 3 minute cycle on repeat for a total of one hour has "broken in" a bright sounding system thus reducing glare and brightness. Similar results with the PAD and Sheffield CD's for burn-in and demagnetizing of every part of signal path which are still available either new or used on Audiogon. This has sometimes helped to avoid expensive, and time consuming replacing of different parts of various systems which initially seemed bright or affected by glare. These items can be used to optimize already good sounding systems, so you won't be wasting money even if you have to make later system changes to get the exact sound you need. In the good old days, there would have been the recommendation to use the Quad preamplifier with its treble shelving control to uniformly and minimally reduce the entire high frequency spectrum, as a quick fix with very acceptable, unobtrusive taming of glare and brightness. Unfortunately, that type of control never caught on. Any equalization is just not mentioned anymore, even when it sometimes has the potential to tame brightness in one second, giving the afflicted audiophile time to troubleshoot until a more elegant solution is discovered.