I'm surprised that 2x10" woofers did not work very well for you as compared to 1x15". You did mention a tube enclosure on your 15" and that may be a reason for more "punch".

It is difficult to get bass sound from the trunk into the passenger cabin of a W123 because of the fuel tank and seats that are in the way. One workaround would be to remove the first-aid tray which results in a nice opening directly into the trunk. This will allow sufficient bass sound to enter the cabin. For aesthetic purposes, you can cover the hole with speaker cloth.

An idea:
Have the woofer box ports feed directly through the 1st aid box into the cabin, using PVC tubes. Whooeeh!