Doug, There's not too many adjustements you can make with Rega arm. Proper alignement, VTA and nothing more to adjust there.Regas aren't the most adjustable arms, but in addition to alignment one can certainly tweak VTF and anti-bias. IME, these are the three tonearm adjustments most likely to affect (distort) the reproduction of sibilants.
My prime suspect is the cartridge, but before dropping money on an upgrade it would be prudent to eliminate user adjustments.
One more possible cause: a dirty stylus. Is the stylus cleaned after each side (preferably with the Magic Eraser method?)