I think I may have solved my volume issue!
What I have done is install some extra speakers
Before I got my 6x9's I had some small eltax concept mini home cinema speakers wired in under the front seats.
These speakers were pretty good and seriously impressed me how good they sounded. They went nice and loud and sounded pretty good but lacked the low end thump so I got the 6x9's to mount under the seat in their place.
The 6x9's gave me good bass but were slightly too big to go under the seats so they ended up in the boot where they sound really good nice and bassy but lacking in volume.
What I have now done is reinstall the eltax speakers under the front seats and wired them in series with the fronts, effectively turning my car into 2 giant 3way speakers
So I have the bass from the 6x9's rumbling from the boot, mid range coming from under the seats, and treble coming from the dash and it sounds pretty good.
I also have the loudness turned on as suggested and it seems to be doing the trick, I shall report back after I get it on the drive to work and get it upto motorway speeds.
Oh before anybody says anything about the home cinema speakers, they are only small (approx same size driver as dashboard) and they are rated 4-8 ohms which is what the head unit puts out, the head unit seems to cope with them very well.
Here is a link to the eltax speakers if anyone is interested
http://www.richersounds.com/product/bookshelf-speakers/eltax/concept-mini/elta-concept-mini-bk