IMO, you pairing bright sounding amplifier to bright sounding speakers, my suggestion is save your money on cable and go try some speakers with soft-dome tweeter like Dynaudio...
Treat the cause, not the symptom!
IMO, 2nd. And the Fire is not exactly lush sounding, if my very minimal experience has any merit.
I admit also, to placing very little faith in significant differences between ANY cables, digital ones especially, so long as the basics of impedances, clean connections, yada yada are maintained. Since the industry has standardized on a 50 ohm connector for 75 ohm interface, .... but i digress down a rabbit hole.
My two cents: you don’t really like your speakers.
One very practical solution would be to tailor a low pass filter (gentle rolloff)and program i into a DSP like Roon’s. Its why its there. I have a switchable, gentle low pas filter* (one insanely good PP capacitor) on all my amps. Most recordings too, err on the bright side. Nothing good happens above 20 kHz!
*say, -3 to -5 dB at 20 kHz. Sounds big, but is very subtle.