It's not too big a job - probably easier than the doors.
First you need to take the sub assembly out - its pretty straight forward, just peel back the passenger carpet under the glove box area and you will see an aluminum kickplate panel - remove that & sub assembly is behind it. Then unbolt & remove the whole sub box.
The amp is buried inside the box, so you need to remove about 15 screws around the perimeter to split the two halves of the box, then the amp just unplugs from the harness assembly & you can replace it,
Are you buying the whole sub or just the amp? I think a rebuilt amp is about $75 exchange - there are several places that rebuild them including Bose themselves (available in the FAQ) or you can replace the caps & transistors yourself if you are handy with a soldering iron for <$20. I recall you can also buy a non-Bose after-market replacement.
Maybe you can try the replacement & see how it works, then get your original repaired for when it goes again.