Do you start fine tuning the system with cables or tubes ?

I have found that cables have a much bigger effect than tubes if you are using good cables.  Changing cables is often as dramatic as change components if you are using good cables.  I've found that changing tubes is much more subtle than changing components or changing cables.

Good cables typically cost at least 10 times as much as good tubes, and the differential is usually much more than that so it's not surprising that cables make a much bigger difference.

This is interesting. Some say that after a certain level tubes make bigger difference while others say the opposite. In terms of cost it can vary wildly. If you buy used cables then even excellent ones while still expensive are not terribly bad. Tubes, well, it depends, right ? Also on how many of them you need.

Tubes make the bigger difference IF your cables reveal them. The wrong cables might obscure them. It's that old synergy thing again. 

@hilde45 - what they said.