I just got a Marantz TT-15S1 yesterday. I checked AVS Forum here to ask my own question. But first let me reply to yours.
I have a Musical Fidelity A308 integrated amp, and Revel Performa M20 speakers. I am using the phono pre-amp built into my amp.
So far, I have found the sound from this combination to be quite nice. I have not observed the sound becoming horribly distorted, except in one case, if I set the cartridge force too high, on the inner track of an LP near the end, it can distort on really loud parts. I reduced the tracking a bit and it got better.
From your description it sounds as though some aspect of your tonearm alignment, tracking force, etc, is not "locked down" so to speak, and is slipping. Could it be when you installed the tone-arm assembly into the base of the turntable you didn't tighten it and the entire assembly is sinking into the base slightly? Just a random guess mind you, but I agree your problem doesn't sound like it is the pre-amp or any other part of the system.
Now, two questions of my own:
1. The stylus / cantilever coming out of the cartridge on my system appears to "bend" a bit toward the inside of the LP as it sits installed on the system. I am not familiar with this cartridge but from pictures, and experience with other phono cartridges, I'ld have thought it would extend straight out. Has anyone else noticed this issue?
2. Any time I try to install the anti-skate screw, even with it screwed all the way out to provide absolutely minimal anti-skate adjustment, it occasionally causes the tone-arm to swing back out and even scratch my record a bit. This is disturbing. If I remove the anti-skate it seems to work fine. As I understand it, when the Anti-skate is set correctly, you should get a nice centerred image. In my case, w/o the screw installed, images are spot on, Sound is great, Any other experience with this? Are some LPs more in need of this than others, etc?
Thanks for any help.