What tonearm cable are you using? You may sibilance as a result of earth grounding/lack/inadequacy thereof.
Have a listen to a Townshend Trough turntable and they cut out sibilance almost entirely which makes you think that you have lost out on treble - in other words it could be an arm issue - try out the isolator as Markpao says
Have a listen to a Townshend Trough turntable and they cut out sibilance almost entirely which makes you think that you have lost out on treble - in other words it could be an arm issue - try out the isolator as Markpao says