Clueless did a very nice job of presenting a great yet brief over-view of the subject AND took the time to provide a vast amount of resources for those that are interested in learning. Kudo's to him for helping to educate those that are willing to learn and sharing his love for tube based audio devices. His post really was "glowing" in content : )
Having said that, I would like to highlight one specific point that Gs brings up. At the same time, this one sentence is phenomenally simple yet highly elegant in terms of the idea expressed:
"But as a rule of thumb, the more output devices means the more careful (read: expensive)the circuit design, since there are more devices it's easier to corrupt the current flow."
If the circuit is not fully optimized, the chances of circuit instability, distortion, saturation of power supplies / output transformers, etc.. becomes more significant as output and number of output devices increase. On top of this, i've found that tubes ( even from the same manufacturer ) have a far wider level of operating tolerances than SS devices. As such, the more tubes that you run in a circuit, the greater the chances for circuit instability / reduced performance levels. Most of this has to due with differences in the internal conductance characteristics of the tubes themselves and stray capacitance within the circuit they are used in. More parts equals increased complexity of both parts and circuitry.
As Clueless hints at, simpler "may" be better but it is all about the individual quality of the design / circuit and what one is looking for / willing to accept as trade-offs in a design and a system.
As a side note, i've found some tube based RF amps that worked better after plucking a few tubes out of the output section. After re-tuning the circuit, power output was up, distortion was reduced, the load on the power supply was lowered, etc.... All of this has to do with the above-mentioned reasons of tubes being mismatched and "fighting each other", stray capacitance, transformer / power supply saturation, etc... I'm quite certain that the same thing occurs in audio based designs that try to achieve "gonzo" levels of output. The more output devices you have, the more closely they have to be matched.
Unless you really are an avid tube enthusiast, i would suggest "keeping things simple" and the tube count down to no more than two to four output devices per channel. Otherwise, the cost and complexity will DRASTICALLY increase above that point. Sean
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