The sub is fitted in front of the ridge on the flat floor under the seat i.e. under the forward section of the seat. 
Thanks for that,just as I thought - might think about Steve P's method as he has dynaudio - didn't you mention
you had heard his - how does it compare to your set up ?
Right, you are going to get me into real sh!t if I get this post wrong.
I heard Steve's system without the sub-woofer and it was not to my liking. Told him so at the time. The sub-woofer has transformed it. No doubt. It sounds smooth, dynamic and very clean. Loved it. No question an absolutely worthwhile upgrade. It is now a really great sounding system. I loved it. He needs to get some good Classical CDs to show it off!

Regarding mine: Well, without the sub there was a horrible upper bass boom with no bass extension. The mid-range clarity was hopeless as the paper coned bass-mid speakers tried to do too much. The sound was too mushy. It was a bit, how should I put it, oh well, fcuked! But I felt it to be better than the Dynaudio for my ears.

+
With my system, and with the installation of the underseat sub-woofer, the transformation is truly incredible. Probably more so than with Steve's system. But this is only an educated (?

) guess as I have not heard them side by side. The bass is very clean and extended. I can hear the bass notes sounding rythmic for the first time and the mid and high freqs are truly clean and quite beguiling. The system has fabulous attack and is very exciting yet musical. The upper bass honk has disappeared completely. But the most significant change has been the unbelievable improvement in treble extension and clarity together with a highly detailed mid-range.
Both systems have been transformed with the sub-woofer installation.
Go for it, NOW.
So this is my answer and you may or may not read between the lines. Both sound really good, at last. But I spent less and got a sub placed inside the car, just where I wanted it.
