Althought I am generally happy with the Bose system in my '00 STS, I thought the bass was inadequate. What I did was to have a JL Audio sub-woofer installed (JL10 IB4) - it's a 10" sub that fits into the existing hole on the rear seat panel. To drive the sub, I had a JL Audio 250/1 amp installed as well. It fit nicely onto the back of the rear seat, clean, and doesn't take up much space in the trunk. (see pics at URL below)
The entire job, install, components, tax, etc. ran just under $600 here in NJ.
Since I have a 92 DTS there is no existing hole to slap in a 10" or 12" sub. I have done the 2 10" box in trunk thing and want to keep all my space. My 6 speaker Bose Gold single cd is very crisp and clear and the 6x9's are almost there for bottom end. I have done a lot of research and have decided to add an Infinity Basslink using the line level speaker inputs. It has a remote gain control you install under dash. I found good prices on Ebay for about $200. I listen to classic rock to heavy metal so I need tight bass more so than boomy. I think this will do the trick. Anyone have this sub with stock bose? I would like to hear how you like it. I was thinking of mounting in the cabin (Lots of room under dash?) Not sure yet.
I would tend to agree with the "FREAK" here......Crisp and clean Bass comes from 8 to 10 inch subs! Not to be mistaken for the idiots that have to have their subs so loud that everyone has to be subjected to their bad taste in music........I'm actually speaking about real sound quality here! Not just a lot of noise. I like my music LOUD! But clear and crisp.
Agree with kcnewell and STSFreak - no need to overdo the bass. One 10" is more than enough to get good, clean low-end sound. Going overboard with multiple subs (especially ones that are low quality) will make the sound worse!
I have a 600w kenwood amp that I can bridge to power a 10" sub. I think a custom install in the panel behind the rear seats would be the cleanest. Does anyone recommend a sub that works well without an enclosure? The Infinity basslink looks like a good solution but since I have an amp already I could get a pretty nice sub....what to do,what to do?
See my posts above, I am quite pleased with the JL Audio (JL10 IB4 component) sub-woofer that I had installed - no enclosure (it's mounted just behind the rear seat panel.
Pics are on the web site.
Also, dts92...you could buy a bazooka tube.. those tend to sound pretty good and you just have to set them in the back and you can switch from car to car if you want
Sub-woofer 
adding a sub 
Free-air Sub 
