The "most foolish" loudspeaker tweak/upgrade ... is

2channel8 ; now that's the spirit of this post. Thanks for providing
this wonderful opportunity to finally get things precisely right. 

The "most foolish" upgrade or audio expenditure is the one that you personally can't hear...emphasis on you personally.

Thanks russmaleartist for the mention of the infamous
"Sucker of the Year' award. The sad fact is there are shameless people 
in our industry, who deliberately attempt to use fantastical claims.   Voodooists that have next to no tech skills,hoping to influence the gullible to hand over money for a charlatans product. From the general postings in this and other forums the vast majority of our members thankfully have no use for these hucksters and fraudsters.

Thanks cycles2 for clear comments--
" Hard to believe some people on this forum are legitimizing Von Schweikert Audio's "ULTRA Internal Wire Upgrade" for $39,700.  If I owned these speakers I'd be insulted by Von Schwiekert coming up with this internal wire upgrade offer.  How did they not get the correct internal wire voiced at design & build time?  These speakers ain't that old to claim that internal speaker wires have made giant leaps & bounds in the past 5 years.  

The real losers are people that own these speakers when they try to sell them as prospective buyers won't want to settle for the non-upgraded version.  Great reason to cross Von Schwiekert off my personal list of manufacturer's that protect the investments of their client"

So well put 1extreme. I think it must be that the same human frailty that's noted 
in "The Emperer's New Clothes" is the one exploited by the hucksters.