What is considered a "standard" percentage of devaluation per year foor high end speakers?

There is no "standard" rate.

(1) In my experience the suggested 40% knockdown in year one can easily range to 50%, and do not necessarily plateau there with no further reduction ad infinitum..

(2) The variable depreciation in estimated fair market value, -- independent of what year out it is-- is also HIGHLY influenced by the state of the economy at the time you pull the trigger. Intuitively, expect a harsher grind-down in gloomier times.

(3) Thirdly, ALWAYS parse out a depreciated value for a dealer trade-in from any proposed private sale value. They don’t equate in any manner. The former is always going to be a much harsher mistress .... full stop.

another variable is how often the manufacturer changes models, or changes a speaker without a model change designation...some 7-10 year old speakers are still current, some have been upgraded/changed a number of times...

I think you also have to look at value from the buyers perspective:

1.  How much shipping am I going to have to pay
2.  What has been the average time a similar loudspeaker has taken to resell...as an example on audigon
3.  How well known and reviewed is the loudspeaker
4.  Do I have a couple of equally interesting alternative loudspeakers that I'm interested in
5.  How much do I trust the seller

These are just a few...therefore, as a seller, you can easily see how your actual selling price might range from 40-70%.

jbl studio speakers hold best value decades. best investment. the same reason i purchased fj-cruiser.

Wow i will be up all night worrying about that.