I looked at the Helikon's specs and see that the recommended load is from 100 ohms to 47k-ohms. The MM section of the VTL is probably loaded at 47k-ohms. So if the cartridge sounds "bright" to you in the highs, that is the probable cause... But that shouldn't hurt anything as long as you like the sound.
This also probably means that if you ever try to use a high output cartridge you'll be back to barely being able to move the volume control, even on the lower gain MM inputs.
This also probably means that if you ever try to use a high output cartridge you'll be back to barely being able to move the volume control, even on the lower gain MM inputs.

