I was reading about your nice table. The review mentioned that the cartridge should be checked for alignment with an alignment template to be certain that it's correct. It also suggested that the cartridge be as forward as possible. If the cartridge is realigned, that may resolve your distortion issue. I'd also reset your ant skate as this could cause distortion as well. I would do all of this prior to calling your dealer or Marantz.
I use an Origin Live Belt for my modified Thorens and it is better sounding than the Thoren belt. As far as belt slippage, if you have around 60% alcohol, I'd clean off the belt and see if that eliminates the slipping. Don't play warped records without a heavy record weight but a heavy weight would probably play hell with your motor.
I have an ASR Mini Basis Exclusive phono stage which punches well above its weight class. I'd recommend finding something along these lines on the used market. That is if you're unhappy with your current phono amp. The Clearaudio cartridge should be good enough. I believe Clearaudio cartridges are made by Audio Technica.