-> So what if anything is true about the underpowered amps causing speaker
-> damage, and higher powered amps exercising more control? And,
-> what is "different" about power ratings for tubed amps than electronic?
The premise of underpowered amps vs. overpowered amps being more likely to
cause permanent damage to a speaker is still true.
Tube amps differ in two ways - they produce even-order distortion products
instead of their solid state couterparts which produce odd-order harmonics, 
and
also by the nature of the design, "soft-clip". The resultant waveform when
overdriving an amp is FAR less injurious to a speaker both in the lack of 
.C.
at output (xformer coupled in most cases) as well as square wave/harmonics
noted to nail tweeters.....
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