Not all tube amps are the same. But, to me the tube sound is thicker and has more weight or heft to the sound. There is also a bit of compression to it. As I stated earlier, I found that the Mesa 800+ sounds almost like the 400+, but the 400+ has more of a meaty or heavier feel to it. For me the trade off for the better tone was not worth it. I love tube amps. But they are heavy and I am getting old. I love a light weight rig.
This is my experience with the Strategy and the D800.
The D800 is a great amp. I agree with others that it sounds reasonably "tubey". Non-linear feel, slight compression, and it has some harmonic "thing" (magic?) going on even when you're not overdriving it.
That said, it's relatively polite next to the Strategy. The way the Strategy unloads on my cab it's like I'm playing a sledgehammer. A really big sledgehammer.
To me, the D800 is a quick and accurate amp. I wouldn't call it clinical due to the things I mentioned above... it feels very good to play through and sounds great, but it responds like a solid state amp; fast and tight.
The Strategy has a totally different transient response and attack envelope. It plays looser and hits harder. There's a "weight" to the sound, like it's constantly trying to overcome inertia and takes a moment to blossom. I can manipulate the way the amp responds based on how I attack it with my fingers so the amp itself becomes an instrument I can play. I don't really get that with solid state because the notes happen so quickly there's not a whole lot I can do to manipulate them.
I really like the D800. I'd recommend it to anyone and due to it's form factor I use it more than I use the Strategy... but it's a totally different animal. I wouldn't buy one thinking it'll perform like the other.