Welcome.
I’m 59 yo and have had tinnitus for 30 years. I’m listening at 85dB peaks 10 feet from my speakers, avg is more like 70dB which is very satisfying and far too loud to talk over if we have company. This is a very safe zone.
I just had an audiogram which confirmed no changes in the past 15 months. However I "feel" like my tinnitus is louder sometimes, and/or a different frequency, sometimes pulsing but just one one side.
I’ve learned to live with it.
As far as I know, there is no way to measure tinnitus so not sure how you came up with 18khz? And audiometers only measure to 8khz anyway.
Anyway, I built my system around my attenuated range of 500hz-4khz. Good midrange from my Harbeth 40.3 XDs and smooth highs. Details in my profile. The key is rich, full, smooth sound, at very safe levels.
Definitely try and warm up your system as others have suggested. I had more issues with harshness on the digital side than analogue initially, but that problem is solved now.
Know that your ears will get over excited and that may also mean increased or variable tinnitus tones, hopefully temporarily, but as long as you are careful with SPLs, you should not be concerned about worsening hearing loss - other than age related.
Hearing aids won’t solve the problem. They may make tinnitus less prominent by amplifying certain frequencies, and some have white noise but they are often oversold for tinnitus.
Lastly, I asked my audiologist about hearing aids and music, and my fear that they will poorly amplify what my speakers put out, and she said some have a music mode that can help PLUS she can drill tiny holes in the membrane that allow more external / unamplified sound through. Not sure I’m ready to drop $6K on that experiment just yet.