Turntable sibilance

"but how do you change the vta on these arms"

There's no adjustment for it. You have to add spacers if you want to change it. It may be a good idea to list your system.

The Michell VTA adjuster available from musicdirect.com works pretty well if your arm doesn't have and adjuster.

http://www.musicdirect.com/p-7336-michell-rega-vta-adjuster-for-michell-turntables.aspx

It should make a difference you can hear. Also have you checked the cartridge alignment with an alignment gauge?

I think the green slider you mentioned is the anti-skate adjustment which should be set to the cartridge tracking weight/force or slightly lower.

Beyond VTA, there are two things that would manifest as sibilance (not having heard the actual rig of course):

Mistracking from either VTF or anti-skate set incorrectly. Since you've tried VTF, then it could be anti-skate (although I doubt it). Try playing individually through each speaker--if you notice a tonal difference (or distortion) through one channel vs. the other, that suggests anti-skate could be an issue.

Phono stage loading--if the loading is not matched to the cartridge, the tonal balance will be affected. Either muddy and closed in, or bright, harsh and sibilant. I think this cart should be loaded between 100 and 150 ohms.

Good luck--I'm sure you'll ultimate get the right answers here in the forum!

Kj...vtf maybe...antiskate...no way.