Tube amp suggestion

Sbank, that's good advice.

I'm considering moving from solid state to tube amps but don't really know where to start. I currently have:
Pair of B&W 805D
Marantz sr7005 (setup as a preamp)
Pair of Rotel amps, bi-amped

As mentioned above, start with replacing the Marantz with a good tube preamp. A lot of the benefits of tube amps will be lost going through that receiver.

I've owned Rotel(980BX or 990BX) amp and preamp, and low-end B&W(301), and while great equipment at their price point, I found their sound together lean and flat. Obviously, the 805D is far better than what I had. But, I tried the Rotel with a few other speakers and still found it to be a little lacking for my taste. Also, I have spent quite a bit of time listening to the 805's at a friends house.

As for the Almarro A318B suggestion, I have also owned that amp for a good amount of time. It is a superb performer far above it's pay grade. I used it to drive 90dB/4ohm Jean Marie Reynaud Twins nkII monitors. The match was fantastic. And there is some unique about the 6C33C SET sound - incredibly transparent, almost no discernible character of it's own unlike other Triodes. The big advantage is that it is an integrated amp, so you wouldn't need to use the Marantz.

But, 18wpc SET will not be everyone's cup of tea, depending on room size and musical listening preferences and average volume.

A safer suggestion would be something with more power like a used Manley Stingray integrated.

Ok, I have an opportunity to replace my rotel solid state amps with a sonic frontier sfs-50. Has anyone had any experience with this amp and if so would it be a worthwhile upgrade. I can buy this for $1000.
What's the upside and downside of tubes vs. solid state?