JBL audio w/ 75% of speakers failing

Bought a 2005 dirt cheap. Fired up the radio to discover that only the driver's door speakers play at proper volume & quality. Balance / fader action verified that the other three corners are muddled and way too quiet / muted.

How do I figure out if it's the amp causing these issues with a single failure (likely) or three separate doors worth of blown / failed speakers? (Less likely.)

If the speakers are all blown, I'll throw a coaxial down in the lower door and delete the tweeters all the way around, then wire those directly to a new head unit.

If it's the amp, then I can save a lot of labor by installing a new head unit and the Metra JBL amp bypass kit, correct?