I ordered
Infinity BassLink SM 8" compact powered under-seat subwoofer enclosure at Crutchfield.com and have test fit it in my w123 sedan. On the passenger side it fits under the seat, but due to a small hump (i'm guessing clearance for the muffler?) it doesn't want to slide in to be completely with the seat in it's rear-most and lowest position, it's still slightly visible, but leaves room for me to sit comfortably. The driver's side doesn't fit as well, though I think that may be because of some excess wiring under my carpet for my aftermarket heated seat kit. I need to make sure that both seats are at their lowest setting and still slide, I think if I pull the wiring out from under the carpet it might fit better. I will take a bit more time to try to fit it better, or I may return it to exchange for a smaller unit. I also considered trying to fit it above the fuel tank, but under the cargo shelf. Possibly bolted on upside down to the cargo shelf. Not sure how much clearance there is there though, I know there's already a vacuum reservoir on one side, and the first aid kit on the other, so there might not be space left.