If you think about capacitors, check out this video

If you suffered from that problem you would have known it by 2004. 

It wasn't so much a counterfeit capacitor as capacitors that used counterfeit electrolytic.  There's a whole story about a chemist who "stole" the electrolytic recipe and apparently didn't do it very well. 

None of this AFAIK has had any impact at all on audio gear.

I’ve had two boards from subwoofers go bad.  I’m wondering if the culprit is the metallized polypropylene film capacitors which are closest to the power source.  Looking at them, you can’t tell that those capacitors are bad, from what I’ve seen.  Any ideas?

No way to know what went bad on the board.  If you suspect certain caps, the only way to test this is to remove the cap from the circuit and test it with a meter that tests capacitance.  You could do a rough, in-circuit test with an ESR meter (equivalent series resistance).  For most home diagnostics, an ESR meter is more handy than a capacitance meter.