w126 thought on subs...
 

1980 280se
anybody put them anywhere but a box in the trunk? i read somewhere about putting them behind the rear seat headrests?
as well as upgrading speakers (i've got 4 x 6's in the front, i think the same in the back)

I don't think it matters where you put them since they operate at such low frequencies. The only problem I can imagine would be vibration.

lookin close, size might also be an issue (i've got 10 inch subs) i was hoping it would be a beautiful fit, but i think those boxes are a little small... the modification necessary is probably beyond my means currently, but it would probably be awesome if done well.

Been there, done that. Problem is, for a sub to work properly and efficiently, it needs to be housed properly. Bass response depends on the sub being firmly anchored and installed in a box of appropriate volume. There are free-air subs, but they generally expensive, don't perform as well and require more power.

So...if you wanted to install in the rear headrest, you would need to construct a proper enclosure for underneath the subs. There IS room, and some people have done some really nice installs in this manner. But it is a LOT of work. So, the trunk is generally the best option.

As far as your speakers, are you sure you have 4x6s? Either way, I would make them the last thing to upgrade. I would upgrade the head unit, add proper amps for both the stock (midrange+) speakers and a subwoofer, and then see how the system sounds. If you still found the system sounding a but dull after making all these mods, then I would replace the stock speakers - but not before. Proper amplification and crossovers do wonders for the stock speakers, which are fine speakers (unless physically damaged).

i am 100% sure the front has 4 x 6's (blaupunkt), don't know on the back. just doing the subs (2-10 inch, wired to a 300w pioneer amp) and head (sony) made a huge difference. not gonna do speakers until i get some spare money (car broke down the other day, getting it runnig is the primary concern, otherwise i just have a really nice room to listen to music in)
i was also thinking about splitting my subs into 2 boxes, and putting one on each side of the car (behind the wheels, theres a spot that looks about perfect thats a littlelowered, thinking it migh be a good way to lock them down.). my primary concerns are getting a sueable trunk and making it look pretty, now that it sounds pretty.

just a suggestion but what about putting some 10's under the back seat. Its been a while since i have looked under the seats but there might be enough room. If there isn't then you could install the shallow woofers.