How to accurately gauge speaker sensitivity to match with tube amp?

@mammothguy I’m not so sure you’re not on to something, as well as the others. This was an interesting article (1967!) which helps tamp down on whatever big debates were happening about DF back then. (Are the same debates happening today?)

It seems to affirm what both "sides" here are saying, namely that while astronomically high damping factors are irrelevant, that higher damping factors can make a difference.
From the article (link at bottom):

"CONCLUSIONS
It should be obvious at this point that the quoted damping factor of an amplifier is important only if the figure lies somewhere below 20 or so. Changing the damping factor from 2 to 20 does change the performance of the loudspeaker system (for better or for worse, depending upon the speaker). But trying to prove that a damping factor of 200 or even more is somehow better than one of 20 is pretty unconvincing because the effective difference in the practical case cited is only that between 1.25 and 1.32.

But someone is bound to insist that exhaustive tests have been made with such and such amplifier and that a very high damping factor is better than one down around 10 or 15. "The bass is just a little cleaner, just a little more natural and open" is the way the argument usually runs.

In a given situation, this may very well be true. Rs is a byproduct of negative feedback. The more such feedback that is thrown into a power amplifier circuit, the lower the generator impedance and the higher the damping factor. The point is simply that if a lot of feedback has to be used to lick the distortion in a particular circuit, fine - use it. But don’t believe that the reason it sounds good because of some astronomically high damping factor."

https://butleraudio.com/damping1.php

Once again I take the opposite view of millercarbon. With tube amps, especially OTLs, the speaker impedance is more important than sensitivity. 

Matching the power amplifier to the loudspeaker is usually a function of driver size, efficiency and budget. Efficiency is not the only determinant – even though they may be very efficient, many speakers with large drivers need tremendous amounts of damping factor and current capability to control the driver properly.

@hilde45 This quote from VTL is largely false. The diameter of the driver has **nothing whatsoever** to do with the ability of the amplifier to ’control’ it (although I will concede this is a popular myth)! What **does** matter is the relationship between the amp and driver.


To give you can example, my speakers at home employ dual 15" woofers, which by most accounts are large drivers. Yet they are easily controlled by our little S-30 amplifier which has a very low damping factor. This is because the designer of the speaker prefers smaller tube amps, and our S-30 has full power bandwidth to 2Hz. The size of the drivers (TAD 1602s for anyone wondering) is irrelevant. That same manufacturer (Classic Audio Loudspeakers) features an 18" driver in their next model up, yet no worries controlling it with the same amp! In fact, our MP-1 **preamp** by most accounts would seem to have an output impedance too high and power too low to be able to drive a loudspeaker, but it does more than a passable job on both of these speakers as long as you don’t need more than a conversational volume level- and the bass is just fine!

So don’t be fooled by the high damping thing- it isn’t real.

Well, it seems that audiokinesis put me in my place. Hey guys, it's just been my experience that woofers are quicker and tighter with a higher damping factor amp.

'Tight bass' is a coloration that does not seem to exist in the real world. You only hear it or hear about it when dealing with audio reproduction. Admittedly, its popular, but probably in the same way that turning up the bass tone control can be pleasing- pleasing, but not natural and not neutral.

@atmasphere re
Thanks for correcting VTL. I'm surprised they're out there with such false information. 
I'm grateful to bury the damping thing. Clearly, it refuses to die, generally.
There may be different senses of what 'Tight bass' means. One way I take it is that a speaker emulates a string bass similar to one at a good seat in a jazz club — clarity in the fretting, the pluck, and a bass-like attack and decay of the note. What I've been hearing from my speakers, at times, is a bass-colored fog. That's at least what I mean by that phrase.

@atmasphere +1

An amplifier, cables and speakers form a system in which they interact.

Anyone who claims one thing in particular is a panacea for all ills is prevaricating.

To wit, from MBL Noble Line N11 line preamplifier @
https://www.stereophile.com/content/mbl-noble-line-n11-line-preamplifier
"Some of my work involved tuning the power supply. It may come as a surprise to learn that you can change tonality without even touching the signal path, because the signal originates from the power supply. The impedance curve of the N11’s power supply is absolutely homogenous from DC to 200kHz, which creates a very balanced sound. I also tuned the resistors for the voltage gain, using a mix of carbon and metal resistors to create a neutral balance. There are a lot of preamps that claim to be ’neutral’ or ’in balance,’ but there are different shades of ’neutral.’ If you have a tube preamp, for example, ’neutral’ is at a different level than solid-state; it’s not better or worse, but it is different. It took a lot of work to find the tonal balance I like a lot that measures well, with low noise, and fits very well in the Noble Line." [emphasis added]

One can only assemble a system in one’s locale with one’s components.
All else, especially idiot topics "What [insert device here]?", is pure folly.