I have a 126 now, and had a 450SE, 116 chassis, for 11 years. In my humble opinion, there isn't a very tight acoustical barrier between the trunk and the cabin if you are talking about bass. Mid and high-end would be a different story. This, I think, is due to the thin package shelf with the 2 large plastic trays.

Depending on your personal preference, there's lots of information in this forum on retrofitting 6"X9", 8", and even 2 10" subs in the package shelf itself. I haven't seen much on installing large sub boxes, but that would be easy, and I have posted a thread on using self-powered sub units like the Kenwood WOOX and Infinity Basslink.

There isn't much depth to the underside of the package shelf due to the gas tank. So, some work is required to get the right size volume for the box(es). Some claim that it's worth the effort. I'm real happy with my Kenwood, and also with the Basslink (in another car, but with a similarly 'sealed' trunk).

There's a principle that I don't fully understand that lets you get away with producing excellent bass in a sealed trunk and hearing this in the passenger area, called 'cabin transfer'. Perhaps another more knowledgeable member might be able to elaborate on how this phenomenon works...

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1986 560SL
2002 Toyota Camry
1993 Lexus