The "tube" sound

[gbarchus]

Discovering the variables that contribute to the development of my sound has been a fascination and kind of pilgrimage. A pilgrimage because, along the journey, I have been distracted, strayed and returned to the path many times. A lot of people talk about tube sound and I'm  beginning to wonder what they mean.  
 
I have been a tube fanatic for years. I've developed an ear for the differences in the characteristics of 6L6/5881, 6550 and EL34 power tubes. I have an F2B which I've used with solid state and tube power amps. I've used hybrid amps that have solid state preamp and tube power amp, tube preamp and solid state power amp and mix or match solid state/tube preamp and solid state power amp.
 
My question is, for you, where does the tube sound come from?
 
When using a mix and match preamp, clearly there is a distinction between all solid state and all tube (which I prefer) and it appears to be in the realm of fidelity. All of this is extremely subjective for me. There may be some audiophiles or engineers who would like to clarify this.
 
For me, though, it's the power tubes (with rectifiers and transformers adding their color) that make the sound. I've gone the route of 1200 watt clean power and there's a lot to be said for that, but give me a tube power amp that I can place right on the edge of distortion/compression/saturation and cross over just by digging into the string a little harder. There's a wealth of tone available at that place.
 
With solid state power I am very inconsistent in getting the right tone from night to night in different venues. But, with a tube power amp (even with a solid state preamp), I can usually dial in my sound every time.
 
Would anyone like to join in and add your perception/knowledge about this?

[son_of_magni]

Ok, I'm not a real expert on this but my understanding is that transistor distortion is in the odd harmonics (square wave clipping) and tube distortion is even harmonics (triangle wave).  And even harmonic distortion just sounds better.  But to make a good sounding high power tube system is very difficult because of the problems with impedance matching.  With a transformer output it's hard to get a low enough output impedance for many speakers.  A 4 ohm speaker is likely to get a boomy sound to the low end.
 
If you really want to learn a ton about tube amps, get this book:
 
http://www.transcendentsound.com/audioreality.htm
 
This guy built the amps I use for my stereo.  150 watts per channel tube amps.

[adriaan]

The real bottleneck in any amplifier is the end transformer. Get a world-class end transformer and life will not be the same. (Don't ask me what an end transformer is, all I remember is that it's the biggest object in most amplifiers.)
 
I used to have a twin EL34 amp (that's low power, I know) with a bad transformer in it. A friend replaced it with a vintage Unitran transformer, then measured the frequency range, and it went way up over 20k, within a few dBs.
 
This was one clear sounding little amp! It was driving a 12 Fane Disco 80W spealer in an undersized closed box, so the sound didn't carry far. It wasn't loud, but it was very, very clear. Sure, it's been gone for a few years already.

[811952]

I think the beautiful thing about tube power is the soft compression primarily and the type of harmonic distortion secondarily.  I play through two 200-watt mono Bogen tube amps when it's a situation where tone is of utmost importance.  Otherwise, they are simply too heavy and fragile to lug around because the transformers are BIG...