Probably just needs a new battery. When a multimeter runs low, it throws everything else out of wack.
You have a few options on the table. Only one of which gets you stock-ish performance.
1: refoam. Use the pioneer foam and hope for the best. It may or may not work. Not all foam is equal, so its hard to say what success you will have.
2: different drivers. This will change the sound. It is probably your most risky option. You could easily end up with something that sounds out of wack. But there is a small chance it can be better. I would say that even if you knew every single detail and spec of your drivers, there is about a 10% chance you could be successful.
3: Just build new speakers. This sounds crazy, but think about it. The pioneer foam is 23 bucks, new drivers would be closer to 30 bucks.
Peerless makes a 3 inch full range driver that goes from 57hz to 20K for 54 bucks a pair https://www.parts-express.com/peerless-by-tymphany-830987-3-full-range-woofer--264-1054
You get a pair of those, you go to home depot and have them cut up some MDF for you. You get a 3 inch hole saw and punch out some holes and glue everything together. You would end up with a pretty bad ass set of speakers for pennies on the dollar.