ok this post is to answer definitively the question - can the drivers be updated in the HK subwoofer to improve it?
So. For those who wish to repeat this themselves, the speakers I used are Rainbow EL-W5 speakers. These arrived today:
They fit perfectly in the harman kardon subwoofer, its just four screws per speaker and the rainbows slotted in fine:
note the woven cone, and the large magnet. I'm not trying to turn into a Rainbow rep, but these are NICE speakers. If you want bass from a 5 inch speaker you need a good one I'd imagine, and this fits the bill.
So does it work?
YES!!!!
The bass response is improved dramatically. A richer bass sound is delivered at the low end, and the punch of the bass if that is what you like is very much increased. I have detected NO port noise induced by this upgrade, and the richness of the sound is detectable even to an untrained ear like mine. Could these speakers sound better in a difference enclosure? Don't know, don't care. They sound great here, and are a vast improvement to me.
I will never be looking to upgrade to a larger sub in my car now - this is perfectly adequate for me. As sound is such a subjective experience, you can only take me at my word, however I invite anyone who is passing my way or at a meet who is curious and I will be happy to let them form their own view by having a listen. Feel free to pm me.
The question is answered definitively - you can improve the bass from the Harman Kardon subwoofer by upgrading the bass drivers inside, a pair of rainbow EL-W5's had done the job beautifully.