I've gone massively against Flash's advice on car audio and installed a DLS RXT6. I've done this purely for the good of the 968 community in the name of research <img src="/forum/images/smilies/968/rock.gif" class="smilie" alt="" />
I bought the RXT6 knowing that its a good little sub so in the worst case I could mount it in the boot or cubby, or just sell it on for a small loss. Best case I could mount it under the dash.
I use my 968 as a daily driver and 90% of the time I'm the only one in it, but I didnt want to upset the other half (she's 5' 11") by making her less comfortable. I screwed the mounts through the mats, carpets and into the plyboard in the passenger footwell. With some good old fashioned shoehorning this is the result:
The camera flash makes the box visible but in normal light with a black interior you cant see it unless you're looking for it. The gap below the sub to the floor on the incline is approx 4.75". I took my girlfriend for a drive after installing it, but didnt tell her what I had done. She didnt notice the legroom until I pointed it out to her.
In terms of the sound, I'm currently in the process of installing some 100WRMS Alpine Type R's so I dont have everything set up properly. The RXT6 definitely succeeds in moving the staging to the front of the cabin. At times it even sounds like the music is coming from in front of the tweeters. The sub is plenty powerful enough for me and I've got it running off a DLS CA51 (160WRMS, RXT6 can handle 300WRMS)) with the gain dropped lower than I need to have it.
Negatives are that sometimes when listening to the radio when lower tones arent present the staging moves back again and it all sounds a bit flat. Also sub under dash will find a few extra rattles for you!! Like I said before, its not all properly 'set up'. The sub is also still in the running in period. I think I can reduce the staging diferences and rattles with a bit of setup work. Also Im planning on running the amp bridged for the front speakers and then the rears off the headunit.
Its also worth noting that the sub can be removed in about 30 seconds in case you do need that extra legroom. Will post up a better review once its set up properly if anyone is interested.