The sound you are referring to is almost certainly on the record, and nothing you can do will ever change that. Compare different copies of the same record and this is one of the differences you will likely find. The improvement you heard is minor improvement in cartridge setup. VTA by the way should always be set by ear. Also you do VTA last, as VTF affects VTA. So if you heard any improvement it was blind luck.
Forget sibilance. It is there, or not, and nothing much anyone can do but buy another copy. Which, this being the nature of vinyl, I can guarantee will have some other flaws.
Instead, listen for the balance between the attack of notes and their fundamental tone or body, the balance between top and bottom end. Adjust VTA very, very slightly up or down. Either way. If you go up and the sound gets too thin then go down. If you go down and the sound gets too fat then go up. Whichever way you go, keep going one tiny bit at a time until it stops getting better. Then go back about half of whatever the last step was. Keep going back and forth until the sound locks in and you know you nailed it.
This is the way to do VTA. When done, you may be higher or lower than you were but it for sure will sound better because it is now set the way it should be, by how it sounds.
