I can't resist. I did read the paper and I find it VERY interesting. Actually I should have been asleep and it's the paper's sake I'm not
To sum up, the paper says: Tubes have strong even order harmonics, in solid state systems those are much weaker (opamps don't almost exhibit any). However solid state systems have strong third harmonics. As the paper states, even order harmonics (an octave higher than fundamental tone) sound musical to us and give the tone "body" - odd order harmonics rather remove it and contribute to a dull sound.
The point of this is that tube amps distort more but that distortion sounds pleasant to the ear. Solid state amps are missing even order harmonics (compared to tube designs) thus missing the... harmonic sounding distortion that would compensate disharmonic taste of third (and 5th and 7th) order harmonics.
The paper states one nice thing - opamp's distortion is so obviously ugly sounding that only 5dB of it are detectable by listeners whereas transistors have to distort 10dB over the fundamental signal to be detectable and tubes may distort even up to 20dB without being noticed by untrained listeners.
That means you can load a tube amp 15 more than equivalent (in power) opamp using solid state one (but I've got a question... aren't solid state amps capable of twice more power handling than tube amps at the same price range?
Microphone preamps ARE often driven into clipping - that's why the study isn't pointless.
However for music reproduction the matter of clipping characteristicsis less important as most of us don't overload our amps.
The distortion present in recording audible as shrill or closed sound character can't be removed by the means of adding more distortion (hovewer a bad recording CAN overdrive your amp - that's where amp design becomes more important - Portishead 's song Dummy is a good example of a bad recording. I hate it
![Stick Out Tongue :p :p](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
)
That's why I think the paper is especially relevant to studio engineers but not to listeners.