…Last time this came up it was about the possible muting effect of Guild's once-popular double pickguards option.
What I regarded as the credible informed opinion was: "minimal effect", but I forget exactly who said it. I think someone like Christopher Cozad or Acornhouse, both of whom are builders.
The reason given was that by far the largest percentage of top resonance is produced in the lower bout below the bridge…
As a youngster I loved the sound of John Denver’s F-50R, and it had the double pickguards. I argued, unfruitfully for years, that the extra cellulose did nothing to help or hurt the tone. So-called purists were adamant that pickguards, in general, were tone killers and twin pickguards were downright sacrilegious. We watched the popularity of the pickguard-less guitar rise, though I am uncertain as to whether or not the “tone killer” argument was ever responsible for the aesthetic shift. Regardless, many players learned the hard way to appreciate the benefit of the pickguard (or, perhaps more aptly named, the strum guard). Avid strummers readily scrubbed their way right through the soundboard. I play with fingernails and an occasional thumbpick, so I don't personally benefit from a pickguard, but I have nothing against one, either.
Wait...have I just veered during my own veer? LOL
To your point, Al: As you well know, the sound of an acoustic guitar is generated primarily from the top (front, face, soundboard, etc.) and the area of the top most responsible for tone, the prime real estate (or beach front property - as Kent Everett calls it), is the area between the bridge and the tail. This is true whether the instrument is conventionally X-braced, or is an archtop, or is a more modern “free top” (such as a McPherson or Batson). It should go without saying that every component of an acoustic guitar contributes to the guitar’s overall sound. However, as a contributor to overall tone, the upper bout region of the top is insignificant when contrasted directly with the lower bout. You can easily test this, non-destructively: Purchase a self-adhesive pickguard and affix it to the lower bout. It will kill your tone. By contrast, feel free to affix pickguards, decals, wood chips, cat fur, etc. to that area of the top from the waist to the neck block, without fear of “deadening” the sound of your guitar. That simply won’t happen.
Addressing the thread: Most luthiers seem to agree that the best sounding finish on an acoustic guitar is no finish at all, but also agree that some finish is better than none as, left unfinished, wooden instruments are simply too susceptible to damage. There has been some postulation that a finish can actually assist in telegraphing the sound waves across the top though, to my knowledge, this has not been established. All things being equal, the finish plays one of the least significant roles in the overall tone of the guitar. At best it is a prophylactic and, in most cases, a beautifier. The quantifiable factors most affecting the (acoustic) tone / sound output of the guitar are:
1.) Design - Size, shape, bracing, etc., by far the biggest contributor.
2.) Materials - Wood, carbon fiber, plastics, etc. Design is far more critical than material.
3.) Strings - Everybody knows that “dead” strings make the finest instruments sound pretty awful.
4.) Finish - Sprayed on, brushed on or rubbed on, finish has some impact on the overall sound, but surprisingly less than most believe.
