Should I Modify my Matrix 804 wiring?

Silver was used by B&W themselves in some limited editions of some of their speakers. Wiring is a weak point of almost all speaker systems. I don't think substituting new wiring would degrade resale in the same way as changing the crossover, etc. It is a passive mod. in that it essentially leaves the speakers just as they were. Call it an upgrade rather than a mod. and you would be telling the exact truth. There are other good wire ( besides silver) available, do some research and go ahead if you are good with a Sod. Iron or know someone who is.

Keep in mind that if you later try to sell them, they will be worth less. People generally don't like to buy modified speakers. (If you can sell them at all).

I see posts all the time that say... "these were just modified a couple weeks ago"..."they sound fantastic"..

This of course means the person was not happy with the results if they are selling them so soon after the mods.

I had a pair of 803's which I modified with silver cable.
In my system midrange clarity improved and the bass was much fuller.
The improvement was not huge but overall I was pleased with the result.

Cheers