Lots of knobs might cause more repair chance

Your friend has a point, but in my opinion its not that big of a deal. Knobs (Pots) have a tendency to wear and/or get dirty. When this happens, you'll hear a crunching type of static noise. In most cases, the pots can be cleaned and the problem will go away. Sometimes they need to be replaced.

There are other issues you can have with older equipment, but I think your friend was talking about the pots specifically.

"ebm double negation product in math gives positive magnitude."

Double spending in real life gives your dealer positive magnitude.