****So my final takeaway on my original question is that you can't achieve any real comparison that means anything objectively.****
Your premise is not quite true. There is value in comparing tables with different arms and carts on them. There are characteristics of sound that are primarily influenced by a turntable's speed stability which can be perceived no matter what arm or cartridge is on it. Rotational stability is probably THE most important aspect of reproduced sound as this affects the rhythm and feeling in music. We tend to focus on frequency response related issues and "detail", and not as much on speed stability. Good speed stability affects soundstaging and our perception of detail and overall tonality as well. I think you are correct that a thorough comparison cannot be made with different arms and cart, but paying close attention to speed stability is a darn good place to start.
Your premise is not quite true. There is value in comparing tables with different arms and carts on them. There are characteristics of sound that are primarily influenced by a turntable's speed stability which can be perceived no matter what arm or cartridge is on it. Rotational stability is probably THE most important aspect of reproduced sound as this affects the rhythm and feeling in music. We tend to focus on frequency response related issues and "detail", and not as much on speed stability. Good speed stability affects soundstaging and our perception of detail and overall tonality as well. I think you are correct that a thorough comparison cannot be made with different arms and cart, but paying close attention to speed stability is a darn good place to start.