What SS amp do you use with your SF Line 3?

Have you considered that it may not be the amp? I know there are differences between the models, but in my SF Line 1 replacing the stock tubes with NOS 7308s changed the sound from similar to what you describe (I called it dry and somewhat brittle) to palpable and lush, though not to the point of being syrupy. It is not what I equate to a "tubey", but it is certainly less sterile and analytical than the stock sound.

It was also not necessary to use NOS tubes in all the sockets. Replacing the pair in the rearmost bank (final gain stage?) had 95% of the effect of replacing them all. At first I had Amperex World Globes in all three banks. Later, hearing about the "rear bank only" approach I replaced the two forward pairs with Sovteks. There was very little change. That this worked so well was important from an expense perspective as it allowed using better tubes without going broke in the process. Let's not even get into the problem of finding a quantity of a particular NOS tube!

If this is old news to you please accept my apology for burning bandwidth. In any case, good luck in finding a solution.

As some of you pointed out, replacing tubes will make a big difference. My Line 3SE came with Siemen NOS which should be million times better than Russian tubes, but I found the sound to be dry and thin at best. I put in some $8 Jan Philip 6922 and it instantly became warmer and fuller, but I had loud popping noise when I switch to mute and forced me to go back to Siemen. But my pre amp only has 30 hours of usage so far, maybe even less, so I am not sure if I am jumping the conclusion too early. It may have 200 hours of standby time, but I don't think standby current is enough to burn in the tubes.

So this Bel Canto thing is that good?

Usually if the unit doesn't sound OK with stock tubes and "needs" NOS it is poorly engineered unit. It's the same thing as I could say that I can play soccer good in Adidas boots but cannot play with Nike.

Stand-by current is good enough for tubes. On the new tubes it is essential to idle the amps even overnight and than start listening. Further-on idle the small signal tubes for 10...20 minutes before passing any signal and before every listening.

Aso try to damp your tubes with the tube dampers to see the result. Placement of your tube preamp maybe essential as well. Make sure it's not located above the power amplifier and mates only with small signal components.
IMO I love for the small signal SS and drive speakers with tubes. That's where you get passive-like transperency in the small signal and the signature of a tube drive.
You might also try Plinius M16 preamp with powerful tube amps such as VTL MB450, VTL MB250, Manley Reference 250 or ARC VT200 that will drive your speakers at ease.

Not a direct answer to your question, but I also changed the rear most set of tubes (put in a pair of RCA tubes) and was very impressed with the improvement. I then changed the rest to Amprex, and really could not tell a difference. I use the Anthem Amp2, and am very happy with the sound.

In this case, Marakanetz, it's a combination of the current production, tizzy-sounding Sovtek 6922 tubes being put into equipment that was not intended to have a "tubey" sound in the first place. It is certainly not because Sonic Frontiers' products are poorly engineered.

Good suggestion on breaking in new tubes.