You may get 120 bucks on trade-in.

I doubt very much Axiom can provide you with a new amp. They're not building them any longer and parts are likely obsolete.

If the driver plays without warbling, it's likely OK.

There is no "typical" subwoofer. I've known folks whose amps failed within a month. I've had my cheap Sony for almost twenty years. It depends on a lot of factors but mostly on whether the engineer selected parts appropriate for the design. A well-designed sub ought to last at least 15 years.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated