Nearly all manufacturers do not advertise/exhibit their product measurements? Why?

Manufacturers (or, more accurately, designers) measure certain parameters of their gear as part of the design process, and then they listen and voice the product to achieve the final result. Selected measurements are used to develop the product specifications, which are summarized in the published manual. I suspect most manufacturers believe the product specifications provide sufficient information about the product performance so that buyers can make an informed purchase decision. If they believed it would help sell the product, then they would probably provide a full set of performance measurements. As you implied, there are exceptions such as Lamm that provided measurements of each amp and preamp they sold, and there are others. There are also designers/manufacturers who take pride in achieving certain performance measurements, such as Cees Ruijtenberg at Sonnet Digital Audio (formerly Metrum Acoustics) [linearity plot from Sonnet’s Pasithea DAC/preamp].

I suspect raw product measurements are sometimes massaged before they are documented in the published specification, and that every single measurement used in the design process does not always make it into the specification. However, when preparing the published specs, the manufacturers probably don’t stray too far from actuality in the event that John Atkinson, Amir, or another entity or publication decides to actually measure the piece and publish the results alongside the published specs. My Aerial LR5 speakers were measured by Thomas J. Norton, and the measurements were published in a review by Michael Fremer.

As examples of why specs may vary somewhat from the raw measurements:

• Amplifier manufacturers like to show their amp’s power doubling when measured from 8 to 4 ohms, so they might take the power measurement at 4 ohms and divide the result by 2 to obtain the published power at 8 ohms, even though the actual power at 8 ohms may be greater than published,
• a preamp’s output impedance measurement may be reported as the single value measured at 1K ohms, instead of the range of values from 20 to 20K ohms, because potential customers may realize the higher output impedance values that sometimes occur in the lower frequencies would cause a low bass roll-off when used with a power amp having a low’ish input impedance, and
• speaker efficiencies and minimum impedances may be reported in a manner to maximize those performance parameters and thereby reduce the potential for lost sales to folks whose amplifiers are not powerful enough to drive low efficiencies or low impedances.

For an on-topic interview, Google "Why John Atkinson Believes Measurements Matter."

If you spend a lot of money on audio equipment, it is helpful to have a basic understanding of specifications and how those might affect successful partnering of equipment.  Examples include how the relationship between amplifier power and speaker sensitivity affects dynamic performance, as well as how input/output impedances, speaker phase angles, and impedance fluctuations with frequency, can affect the successful partnering between equipment.

I do not agree with the statement that specifications are "pretty much useless unless independently measured".  I cannot remember any cases I have seen with wholesale deviations between published specifications and published measurements by a third party, such as Stereophile.  I would not purchase expensive audio gear without reviewing the specifications as due diligence to verify the gear is likely to serve the intended purpose. 

Power cables and speaker cables appear to be easier to design

Geometry designed to lower inductance (i.e., twisted, star-quad or braid) and sufficient gauge to conduct the current should get you most of the way there with power and speaker cables.  However with SCs, and perhaps irrationally, I have found a sonic preference for multiple, individually insulated, small diameter solid core wires (e.g., Harmonic Technology with foamed PE insulation or Cardas with individually enameled wire).  OTOH, I also like the sound of my vintage Western Electric wire as both speaker cable and power cable, and I am using 7awg WE wire to power my large monoblock amps, which supports my thoughts on geometry and sufficient gauge.  

"Just listen to it" sounds good, but those of us who were around when amplifier power "specifications" were mostly a figment of the manufacturer's imagination should appreciate today's more accurate power reporting requirements.  Is there still a little monkey business going on, maybe some, like reporting half of the 4 ohm power measurement as the 8 ohm power output (when it is really higher) to make it look like the amplifier power doubles into 4 ohms, or maybe the actual distortion at the reported output or reporting results at 1K Hz instead of from 20-20K Hz, but mostly the reported values are much more reliable than in the 1970's.

In the case of speakers, I don't know why any potential speaker buyer wouldn't first look at efficiency and impedance specifications, just to make sure the speakers are likely to mate well with their amplifier.  DAC output voltage is another useful specification/measurement when determining the gain (or not) needed from the partnering preamp.  Output/input impedances for source components, preamps, and amplifiers are nice to have in order to head off potential component mis-matches that can lead to rolled off or uninspiring sound.

I have been running through my preamp options here ranging from purely passive, to a high quality op-amp active stage, to a tube active stage with NOS Amperex (Holland), to a well-regarded unity gain solid state model. The preamp has been sort of a sticking point for me over the years and I have owned around 20 highly regarded (by reviewers) preamps, both tubed and solid state, and at varying price points up to and in a couple of cases exceeding $10K.

The recent comments here about the Benchmark LA4, caused me to reconsider that unit that I was previously interested in but for whatever reason never pursued. Based on all the "professional" reviews, as well as user/owner comments here and at Audio Asylum, it seems the LA4 measures superbly, and drives whatever signal is provided accurately, clearly, and without embellishment. Theoretically, that should work great for me since my front end (streamer/server and DAC) is known for being full-bodied and tonally dense, and my amplification is known for being musical - IOW, I do not need a preamp to add anything to the mix but only to provide a display, a remote, and to stay out of the way sonically.

The only negative I read from a minority of the reviewers is that in some systems they did not perceive the same level of body that they heard with other preamps, and particularly tubed preamps. A couple of people thought the Benchmark made their systems sound a little thin. Of course, Benchmark indicates this is likely because of improved timing and the absence of distortion (and the measurements support that assessment).

In summary, the excellent measurements (and reviews) resulted in my purchasing the unit but how it sounds with my gear, and compared to my existing preamp options, will be the deciding factor on whether I keep it. If it causes my system to sound thin, then I will not be keeping it since I have several other preamp options that sound really good.

@melvinjames +1

Not unlike missing a shot at one end and then running down court and committing a foul at the other end out of frustration.