What does a tonearm contribute to the sound of a turntable?

I agree with ebm - one of the biggest upgrades I have made was a great tonearm - 
Basis Vector 4 for my Basis table.

hi,
20-30% (at most), most important part is the turntable itself, a stable platform is the foundation. 

Tonearm must provide a stable platform for the cartridge to function optimally. The corollary is that specific tonearm/cart combo matters greatly for sound quality.   Cartridge compliance specs help determine tonearm mass needed for good performance.  Mass and rigidity are two key attributes. In general you want to optimize the electric signal transfer from the stylus as it modulates while tracking the record groove. Tonearm provides the platform for the stylus to move and perform that function accurately.

I have a version of damping tape on the drivers in my Spatials. Their previous owner put it on and said it really helped control the sound. I took his word for it.

A few more things:

1. I'm going to re-drill my armboard; I'm off by 3.5mm on my Feickert and when I used Stevenson alignment, I feel the connector wires from the head shell to the cartridge are too smushed together.
   
2. I conducted a decibel test using the Ultimate Analogue Test LP left and right azimuth and found that my right speaker tests about 10 decibels higher than my left. Yet when playing Dire Straits' BIA 45 reissue and comparing with with the SACD, the instruments were in their correct channels and had comparable loudness levels. Waddup with that? I plan on swapping tubes on my Pathos tonight to see if that's the culprit.
   

Btw - everyone's advice and straightforward info is much appreciated.

Waddup with that?

People gonna flip out Simao but I will tell you waddup widdat: azimuth is one of the biggest waste of time specs in all of turntable setup. I have yet to meet anyone who thinks its important who even knows what it is. What I mean is they say they know, but then they get it wrong. If you want to really know go to YouTube or Soundsmith and let Peter Ledermann explain it to you. He totally nails it. As you would expect.

The #1 thing to keep in mind with everything having to do with turntable/arm/cartridge setup is all the specs, all the alignment, VTF, VTA, all of it, is intended to get you in the ballpark. Well except for overhang. That one you want to nail. All the rest you fine tune by ear.

Azimuth does matter, and Ledermann explains beautifully why. But if you listen carefully you will understand just how complicated it is and why it is that you can measure so bad yet sound so good. What you hear always- always- takes precedence over what you measure.

And now if someone who thinks azimuth is so important will please explain what it is and why it matters and get it right we will have a first for this website.