Sound traps/acoustic panels to improve the sound

I need advice:

I have open-baffle Concentus Lyra speakers manufactured by Nightingale (Italy). They had marvelous sound, especially with jazz, but when I moved to a new flat, I was aware that they lack HF extension as if covered with a blanket. The amp is Cary SLI-80 (the other amp is the older SLI-50). They are positioned against the short wall opposite the balcony and placed nearly in the middle of the room.
The manufacturer had told me before that these speakers do not require any specific placement or room treatment.
My question is: should I look for any sound traps/acoustic panels or other room tweaks to improve the sound? I have other speakers on loan now (the cheapest Montana DPS speakers) and they sound very balanced, though do not reproduce jazz music as well as the former speakers. With the former speakers, I was surprised to hear the cymbals when I moved close to them (to high frequency drivers), while I don't hear them from my listening spot (which is not the case with Montana - I can hear everything from my listening spot).
I will appreciate all thoughts and recommendations.
Thanks to all for great comments!

The room is small, 18 sq.m (6 x 3m), the ceiling is approx. 2.7 m high. The speakers are placed approx. 2.5 meters from the short wall against the draped window. They have a radiating back panel, but the back grille is not removed. You may see their pic at To check the HF driver is a good idea, for which purpose I have to unscrew the driver. Duke's and Herman's recommendations to deal with disperson instead of absoption make me feel that these speakers were better suited for my old apartment (which was brighter and less absorptive than the present one). I will explore this further. Anyway, I am waiting for new speakers to arrive.
RPG Skyline really helped my speakers sound more extended in the highs. I swear by them. Absorption can be tricky and have the undisired effect of sucking all of the highs out. Only complaint with Skylines is that too many can make the speaker sound bright.
I recently received Silverline Sonata-III speakers and the aforementioned unplesant effect disappeared, though I admit that the room may be overabsorbed. Even with Sonatas, the highs are not as prominent as could be expected from these speakers. In any case, they smoked the former speakers hands down.
Hi Transl,

Maybe this can help, too:
PZC / Pressure Zone Controller pictures:;topic=47451.0;attach=32146;image;topic=47451.0;attach=32147;image;topic=47451.0;attach=32148;image

Room Tunes:;topic=47451.0;attach=32143;image;topic=47451.0;attach=32144;image;topic=47451.0;attach=32145;image

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