There's no actual rule or value that will always work, it's song context dependent for each & every track actually.
What type of EQ filter are you using to make that cut arounfd 14.0 Khz ?
This will also have a serious impact depending of the way you're altering the sound. If you're using a Low pass filter to filter frequencies above 14.0 Khz, then also check the slope value of the filter. Some EQ allow you to use 6dB, 12 db, 24 db or even more slope settings, the higher the value is, the more signal you're cutting off.
You could also try to combine EQ curves ( or add another EQ after cutting off ) and add a resonant EQ curve ( more like a high-shelf to boost the frequency you think it's disappearing after cutting off ).
But again, check the sound source and also Fx chain on the track(s), if you find yourself to spend too many time with the EQ ( unless it's for sound design purpose ) then something else might be "wrong" with the way the sound is engineered in first place.
Gain stage and well balanced levels between tracks is also important as well as the "energy" taken by each sound in the all spectrum across the entire Mix and that's what caught my attention when reading the last post:
Layering 2 synths / 7 voices OSC / Filter cutoff open around 85% ? --> well, you already got a wall of sound here. Push those tracks in level and it's gonna be very difficult to mix anything. Use a frequency analyzer to visualize what part of the spectrum this layered pad is occupying ).
Don't let you ears full you or let yourself caught up by something sounding gorgeous & massive, especially if you're at the beginning or middle stage of making a full track because then you'll just find yourself missing energy as soon as you make some changes and it's gonna be terribly difficult to mix your song.