Grab a test record with low frequency resonance tones and figure out the static compliance of the cart with the dampener engaged. If it's *way* off - which for ATs is probably anything under 20cu - you might want to consider getting a new cart.
BTW, some people recommend not using any cart with a suspension over 5-10 years old, simply because the rubber can wear out that quickly. I don't think anybody's provided any concrete evidence one way or another, but you might want to keep some kind of beater cart around to swap back and forth with the 150MLX, in case the cart is causing real damage and you need to test with another cart to be sure.
FWIW, I never experienced any sibilance with either my 440ML (the original! not even the MLa!) or the OC9 on my 1200.
BTW, some people recommend not using any cart with a suspension over 5-10 years old, simply because the rubber can wear out that quickly. I don't think anybody's provided any concrete evidence one way or another, but you might want to keep some kind of beater cart around to swap back and forth with the 150MLX, in case the cart is causing real damage and you need to test with another cart to be sure.
FWIW, I never experienced any sibilance with either my 440ML (the original! not even the MLa!) or the OC9 on my 1200.