I've narrowed it down...

Unsound, The 3rd harmonic is one of the lower orders and is the only one considered musical with relation to the fundamental by the human ear. So Nelson's comment is spot-on.

When Crowhurst was writing, it was not generally understood at the time that the odd orders (above the 3rd) were the distortions that the ear finds unpleasant, although you will see Crowhurst refer to 'listener fatigue' with respect to the use of feedback. IOW he knew something was up, but some of the aspects of the human ear simply were not understood at the time.

Charles1dad, the DHTs are far more linear than tetrodes or pentodes. In fact all triode tubes (whether directly-heated or not) are quite linear, enough so that is is fairly easy to build an amplifier with them that does not need feedback. Triodes in general will generate distortion that favors the lower orders while the tetrodes and pentodes generate more distortion and also more of the higher orders and so tend to sound harsher, thus the need for feedback with them.

If you speak to almost any SET designer, they will likely tell you that the lower orders that their amps tend to make are not objectionable to the human ear, which is true. They do color the sound though- the ear hears distortion as tonality- so the lower orders contribute to the 'rich' or 'lush' sound of SETs.

Atmasphere,
I 'suspected' true triodes are more linear thus they don`t need to rely on feedback.Thanks for taking the time to explain the instrinsic differences between triode and pentodes, very educational.
Regards,

When I was listening to my Technics receiver driving my JLB L19s in college, who knew that one day Class A, Triode, OTL, No NFB would one day meaningful to me:)

I was doing more research prior to purchase today and saw that one manufacturer recommended an hour heatup of the tubes prior to listening... Are they all like that? Is there a down side to leaving the amp on (other than shorter tube life) which brings me to the next question. What is the expected tube life?

They are all like that, but 15 minutes is more than enough to warm them up unless you really want to get paranoid about it. Tube life is very dependent on the tube used and the ciruit it is in - some output tubes in some amps will go 2,000 hours, the ones in my Music Reference RM10 MKII are said to go 10,000 - you need to ask the manufacturer of your particular amp, and the input and driver tubes usually last longer than the output tubes.