Has anyone heard the Ref 3?

Agaffer

The initial tube they used was also very decent. The 6550C merely adds a bit better bottom end

Guidocorona,
In answer to your question, no. I first heard of the possibility of changing the caps and the one tube on this forum when Oneobgyn mentioned that he did it. Since changing the tube didn't require sending in the preamp or making any modifications I tried it first.
I still haven't come to a final conclusion on changing to the 6550C tube. The one I tried in my preamp made a difference, all for the bad. But, I listened to the REF 3 at my dealer that had been modified and when we changed tubes back and forth I couldn't hear the least bit of difference when changing tubes. I can't comment on what changing the caps did, if anything. I guess that I should take the 6550C tube that I tried over to my dealer and see if it has the same negative effect on his preamp that it did on mine.
Originally I thought Oneobgyn had to be out of his mind. Based on what I heard when I tried the 6550C tube. Now, I am leaning toward I tried inserting a bad tube.

The bypass caps make for a slightly better upper end. It is worth doing.

I can only make a guess, but perhaps the 6550 has a slightly higher gain than the original power supply tube. In both Ref 3's I listened to, all with original layout, one powering a Rowland 302, the second a Theta Dreadnaught, I had to raise the volume to 70 and 76 respectively to obtained the realistic sound level I am used to with my old LS2B with volume at approx 43%. Not a big deal though, because above 78 volume became too loud on the Ref 3 anyway.

Guidocorona,

It did come with the 6550 and the caps.

It is without question the best preamp I have ever heard. I have it hooked up to an ARC VT 200 driving Maggie 1.6's. I listen to phono at about 72, cd at 65.