Kinda depends on what your goals are; what your problems have been; and what type of person you are.
The cheapest route is always to have BrianK replace the caps in the 3 amps .. and also in the head unit if that is req'd. This route assumes some DIY skills on your part in taking the door panels off; removing amps; re-installing; etc.
If you don't have a hate on for BOSE .. and don't want/need to replace the head unit .. then next best is probably to get yourself an 3 or 4 channel aftermarket amp; the SOS wiring kit; and an amp installation kit (new fuse; 4 guage wire; connectors; etc). This approach essentially converts the system from distributed amps to a central amp design but keeps the head unit and door/sub speakers (although you need to bypass the OEM amps to do this). You'll also need the DIY skills mentioned earlier plus some wiring skills; and you'll need to find an acceptable location for the amp. Easiest location is behind the drivers seat on the firewall panels as this is the shortest runs to Gnd and power in engine compartment. Others have installed in the trunk but that will require also running input cables as well as speaker outputs through the firewall that you don't need to do with the first suggestion. If you just want more base, you'll still need to buy a mono sub amp and mount/install it. There are a couple of people on Prime selling replacement sub enclosures for the OEM location that can handle larger subs but some of these can reduce the passenger footwell space so understand the tradeoffs. BTW .. to fully convert to central amp you'll need a left channel; right channel; and one for the sub .. and if you want to preserve the center speaker, you'll need a separate channel for it which is really just a merge of the left and right channels. And, depending on your needs, you may need to bridge two channels together to get the sub output you're looking for (assuming you replace the OEM one). So, 3, 4, or maybe 5 channels. An Alpine PDX-5 D class amp is an example of a multichannel amp that would work.
If you have a hate on for Bose .. or are just one of those people who feels you have to have some particular brand of speaker, then SOS makes speaker enclosures for the doors that are roughly the same shape as the originals but will accept a wider range/size of speakers; crossovers; etc. But you will still need a new amp setup .. see advice above.
And if you have deep pockets .. and/or want to re-do the head unit to include video/nav/etc .. or are one of those who has to have a high-end custom installation to win shows, then that's a whole other kettle of fish.
