I'd just like to point out that not all tube amps sound "sleepy" or "slow." In fact, that's not a function of the tubes but the amp design itself. It's a very common myth that tubes make everything sound warm and mushy. That's not true. In fact, there are tube amps that sound snappier and more detailed than a lot of solid state amps. So in short: you're not hearing the tubes so much as the amp.
You probably know this eke, but I'm just posting it for anyone lurking who might see your impressions and think "yup that's tubes." These misconceptions have only gotten worse over the years because of the huge influx of cheap Chinese tube amps. The Chinese specifically love the "warm / euphony" type signature for their cheaper gear. Also you have people like NwAv's minions who scream bloody murder about tubes, claiming they add all this distortion.
Never mind transistor mist.
Tubes are actually more linear than transistors, believe it or not. It's transistors have a tendency toward higher distortion. Surprised? I was. Audiophile myths are crazy. You'd think tubes did nothing but introduce junk into an amp's sound if you listened to some of these people railing against them. Good tube amps tend to be more expensive. Some people have a knee jerk reaction when you suggest cost = performance. Of course, there are plenty of solid state amps that are better than a lot of crappy tube amps. And not all expensive tube amps are better; some expensive tube amps SUCK. Like, really suck. But yeah, some of the best amps I've ever heard are expensive tube amps. So people think that must be a sham because they've read some papers online and fancy themselves experts and live in this constructed fantasy where anyone spending more than they do is a fool who has been "tricked" into thinking their tubes are speychul when in fact they're just paying more for amps that are adding junk into the signal.
Plus there's this weird push among some publications to association musicality with highly colored sound. Like, a bloated boomy bass is supposed to be musical or something. I guess.
Plus you got the influx of cheap-o tube amps that sound like crap. It's the amp's fault though, NOT the tubes!
Tubes or not, many of the very experienced amp designers I've talked to on the subject agree that measurements are very important but not the end-all-be-all. They listen. If a design doesn't measure quite as well as another but sounds better to them, then they opt for that latter choice. Of course some amp designers will disagree and always go with what measures best. These amps tend to sound a bit... tight arsed, however. Too flat... not in terms of FR, but in terms of dynamic character. Two dimensional. Musically necrotic.
Yes, there's that dreaded word: musicality. It's been abused so much it has next-to-no-meaning now, but eh. I think either extreme is ridiculous: either making an amp sound like the Dead Sea or making it sound like marshmallow fluff. And once again I'll remind you that I'm not talking about solid state versus tube necessarily, but just different end goal philosophies that can be approximated in either way to varying degrees. There are plenty of solid state amps that sound like marshmallow fluff, and plenty of tube amps that sound like the Dead Sea.
It's all just differing methodology. The whole "wire with gain" thing is a load of BS too. There is no neutral amp. "Neutrality" is a moving target. A convenient reference point for relative comparisons.
Wow, interesting posts! Tubes having less distortion than transistors goes against everything (admittedly: not much) I've read on that subject. However, distortion should be measurable and as such not subject to audiophile myth. Like the unusually high distortion on those FAD Heaven A/C/S models, that Rin measured and which reminded me of some people saying they sound "tubey" earlier in the thread. So it seems that's based on a misconception (of tube sound). Call me intrigued.
As for NwAv's minions, aka the "objectivists" and their ongoing feud with the "subjectivists", I tend to believe that it wouldn't hurt for both sides to look over the fence from time to time. Just like your example of "neutrality", the line of demarcation between science and subjectivity has been a moving target throughout history, and ever so often an idea has started out as an outrageously subjective misbelief at its time and ended up as a major scientific advance a few years (or sometimes centuries) later.