I use the pht's. Micheal Fremor did a review on them. They come with 2 in a pack. The current ones are green and black. The black one sounds the best and beleive it or not really improves the sound. Better soundstage, clarity etc. I wouldn't go without it.
I am using this on a Lyra kloes on a VPI Classic 4.
I use the “Blue Velvet” on my Lyra Delos.
It makes a big difference in soundstaging and natural flow. Things sound more real, less electronic. I have no idea how or why they work. But have tried numerous comparisons with and without on my cartridge. The PHT stays.