So, I added subwoofers to the factory holes behind the seats. Since we don't have 9" subs here in the states, I used 8" with spacers. I'm still not getting that decent hit like I should if I'm using a traditional box. I'm pretty sure it's not ealed that great back there, what else can I do to seal it enough to get those nice bass hits?

 

When I took out my back seats to put new rear speakers in I saw that hole for the sub... Have been planning on doing exactly what you did. So where did you run the wires off of to that speaker you put in? The problem of that hole not having a dedicated box back there is something I was gonna tackle before I put a speaker in there. I was gonna dyna mat it up and try to figure out where to put a port coming out of that enclosure towards the cabin of the car? What setup do you have? Head unit, amp, speaker types, etc?

 

I didnt have those speakers either. I bought used S60R speakers as they factory fit, and I already had the wires and connectors there under the rear deck. The only modification I had to make to make them fit is cut the holes into the rear deck. I recommend checking, you just have to pull a few clips to get the rear deck out. Theres a video on youtube that shows you how. The underside of the rear deck even had the perforations for the speaker holes so it was a perfect and easy cut with a utility blade.
Mine is not sealed very well either but I still get very good bass from those rear speakers. I have rattling issues, but it can only be heard from outside of the car, because the music hides it. I love it its awesome!

 

I'm a little late here, but want to share what I found in the 2004 C70 accessories catalog: