Power conditioning potentially and it depends on the quality of the power in your house/neighborhood. I see a 6% variation in power in my neighborhood and turning on my power amps or my AC kicking on causes lifts to dim. I have crappy power so for me it is important.

I am interested in cables but in an entry level system, upgraded cables should be something like Audioquest Golden Gate or Chicago and I would only focus on unbalanced, RCA type cables.

Upgraded speaker cable or balances cables in this level system will not deliver significant changes in sound.

Components hold there value till they are discontinued or refreshed. I bought a pair of 10YO mono-blocks from VTL for $3300. They were $14,500 new and a used pair of the current edition would be $7K - $9K. Anything that is current, very reasonable that you will get 50% to 70% on resale. Once discontinued assume 25% to 35% unless it is the rare item that is discontinued and consumer demand is high like Oppu Disc players