Unibox AVRs are a distinct and significant down performance compromise from quality build 2-channel separates .
Your post had me a bit confused .,, the title refers to “Tuners” “ but the OP body is a complaint about an irritating wandering volume pot control issue. Are you asking about a dodgy build creating an irritating wandering volume control pot issue?
if it’s a wandering volume control problem in your current unit, , then buying a bottom-level new unit from COSTCO at a stated $400 rock bottom level pricepoint strata Is a risky blind gamble for any improvement at best - these are all built to their pricepoint.