I hear you len, but neither of what we have said is incorrect. You possibly will get a fuller sound from having 6x9s, but the sound stage will be brought back, ie we are both right. I see it as a question of quality or quantity. In my mk3 iv just got the focals amped in the front and a sub and it sounds awesome. Its loud enough to damage the ears but good quality too and it sounds best exactly where im sitting.
I would also say youd be best to amp the front speakers no matter what hu you had, unless the amp is crap and less powerful that the unit, then theres not a great deal of point in having it atall. throw some decent comps in the front, amp them up and see what you think when running them alone with the sub. I certainly wont be going back!