I am probably putting my foot in my mouth, but I think 3dB is correct about belt versus direct drive.

Technically and theoretically, anything you do to isolate the platter from noise and vibration can only improve the TT. Most belt drives still have the motor connected to the plinth and the platter is connected to the plinth so, even in this case, the concept is not perfect. Besides, many direct drive turn tables have a noise level of greater than 80dB, so I don't think direct versus belt is an issue.

The tone arm is another matter and, not knowing better, I think which one is better is a matter of synergy with the cartridge compliance.

PHONO Cartridge Compliance, Tonearm Mass, System Resonance, Loading of MC and MM Capacitance, RIAA Characteristic

A heavy object needs a strong spring. Likewise heavy arms are only suitable for cartridges with a sturdy cantilever. In other words: a cartridge with a low compliance figure needs a heavy arm in order to obtain a fundamental resonance in the region of 8 to 12 Hz.
A lightweight arm needs to be matched with a supple spring, in other words a very compliant spring which is the cantilever of a high-compliance cartridge.


After that, it's a matter of matching the correct preamp with the cartridge. This is also covered in the link above.

As for manual versus automatic, I assume that an automatic has added mass to the tone arm assembly, so that is a compliance issue and can be adjusted for with the correct choice in cartridges.

As for which "improvement matters the most, I'm guessing that it's the cartridge followed by the tone arm, but the best cartridge may not be the best if the tone arm compliance is incorrect for it. After that, I think the preamp is the next step and the same thing applies, like the tome arm/cartridge, the preamp has to be correct for the cartridge in use. I'm guessing here!

All this sounds logical (to me), but as JohnMichael posted, changing his motor improved the sound of his belt driven table, which I wouldn't think would be that important. I have no reason to dispute him, so that leaves me wondering.