Having done two different sub options in the W124, I feel like a bit of an expert.

First, the space in the seatbacks really isn't large enough to get a good box for the subs to work from. Getting all the pieces and parts stuffed in there, AND making it look good, would be extremely difficult I think.

As has been documented here time and again, the challenge with trunk mounted subs is that the gas tank is in the way, AND the trunk is very well sound insulated. In a 300E, I had two 8" free air subs installed underneath the headrests. 6X9 grills were installed where the headrests had been, and a fairly large board was installed under the package shelf to mount the subs and amps to. The headrests themselves had to be removed completely.

In my current 400E, I have built a sub box with 2 X 10" subs ported through the First Aid Kit shelf. Each sub works off about 1 cubic foot in volume. I had about the same power in both applications - a "300 watt" amp, which is probably generously rated 75 watts per channel in stereo.

The free air sub worked better overall. It hit really hard with all kinds of music, and deep bass was much better. I would give it a an "A-"The fact that the drivers were working directly on the air in the cabin I think was the biggest difference. My current set up with the box ported is quite good. I have left the 400E's existing door mounted subs as is, and they do a good job with mid bass, while the sub box does a pretty good job with deep bass. I would give it a "B". The free air subs required quite a bit of work under the rear deck, and major cutting of the space behind the head rests, which were then useless. My 400E has two 3.5" holes for the ports under the First Aid Kit you can't even see. You do lose some trunk space with the box, whereas the rear deck mounted subs left almost all trunk space available.

There are a couple of other good options on this site you should research, including the one from "Italianbenz". Good luck! Mike

__________________
Mike in Denver
'93 400E Now DEAD - Rear ended and totalled.
Replaced by '02 C32 AMG - FAST!