Fine tuning the system. Where do you start and how do you proceed ?

For me it’s all about listening to the music and not being too analytical, and I’m an engineer. But tweaking is a fun part of our hobby.

I love the sound of tubes but I don’t trust myself or hype to pick them for my setup. I have always gone to Brent Jesse at audiotubes.com for his recommendations. It’s amazing the experience he’s had with a tremendous variety of equipment manufacturers. He gets lots of customers feedback on the success of his recommendations. What’s interesting is the best sound in my equipment does not come from “names”. And the preamp tubes have a much bigger impact than the power tubes, but you already know that.

Just my experience, but I don’t know crap

Agreed about preamp tubes, but driver and power tubes can make a big difference too.

I have to say for me it's the power source. The are some excellent advice here today.

Don't change power cables because doing so well do NOTHING to the sound. It's fake news. And if you've got 12AWG OFC cables, no amount of money can but any cables that will ever sound better, or different. Again, it's all marketing. All of it. Anyone who says otherwise is part of the problem. 

@squared80

Ahhh, not true. Power cables can have a very large impact... in particular on power amps. This comes from my extensive experimentation and experience over several years with at least nine different high end power cords and original equipment and also some low end power cords. I did not find low end cords of any value.