I'll start with your questions regarding the Sonicaps.
I'm interested in knowing if the performance differences are measurable, and if so, what are they?
There are a lot of ways that caps are measured beyond capacitance, and many of these are easily and rotinely measured. Off the top of my head I recall dielectric absorption, dielectric constant, ESR, DF, self inductance, temperature ratings etc. Less tested but other known differences might include the thickness of the dielectric, the thickness of the conductive film, composition of the material used as the dielectric and the film, termination methods and materials used, etc.
It the standard measured areas, the Sonicaps perform quite well, but this is not where the main audible differences lie.
Two caps might measure very much the same in many ways, but one might use Tin foil while the other uses Aluminum foil as the conductive material, and they might have a considerable different character to their sound.
In your speakers, are there improvements in phase?, frequency response?, does it have any effect on impedance?,.....if you had 2 or 3 crossovers with all similar parts values, but different brand caps, resistors, inductors.....how do they differ?
You will not see differences in those areas from different types of components. In a since you would be measuring the wrong thing. The right thing to measure to quantify the audible differences are not that easy.
Think of a cap as an energy storage device. As a signal passes through it, it is temporarily stored and then discharged. Low storage time (fast discharge) equals minimal signal disruption and coloration. Longer storage time (slower discharge) will tend to smear the signal a little. It will drag it out over time slightly and run more together.
Are there concrete emperical differences, or are they subjective?
How about yes, and yes. Some differences are easy to measure and some are not.
I can remember spending a considerable amount of time A/B comparing three caps. All three caps were .1uF by-pass caps. They were all three made by the same company and were identical in every way accept for the way they were covered. One was wrapped in what looked like tissue paper and the ends were exposed. The second one was wrapped in a thin poly jacket and it too left the ends exposed. The third one was wrapped in a poly jacket and the ends were sealed. They all three sounded different, and the manufacturer knew it. The manufacturer called the one wrapped in tissue paper the "hot rod" version. It did indeed sound the best.
Your observations of the changes regarding speaker burn in were very typical.
As you obviously would have access to similar speakers with many hours on them, if you were to measure a pair of well burned in AV/ 3's and put them up against a fresh pair, ( allowing for the drivers to break in a bit) what might the measurement differences be?
You won't see changes that will show up in frequency response other than a possible change in the bottom end of its range, but not much. This is just due to a loosening of the woofers compliance.
Steve, You are welcome here as long as you remain civil. An exchange that I had with you on another forum several years ago prompted this response from the moderator to you...
Be aware that if you start calling names yet again, with yet another inmate, such posts will be deleted, and I will be forced to submit your name for consideration to be permanently banned.
Since you have been argumentative with me in the past, my tolerance level for such interactions will be quite low. Positive and constructive posts are welcome and could be a refreshing new start.
Also, I don't want to get into an analysis or debate of human behavior and perception (or anything along those lines) in this forum. You are however welcome to share your own experiences just don't attack any others for their experiences.
Cool?