Hi Mick ,
I would start with the speakers , the MA R652,s although good are the weakest link and better speakers would bring about the most cost effective improvement .
My recommendation for speakers would be a second hand pair of PMC FB1 or FB1+ floor standing speakers they are quite efficient at 90 db and would work very well with the low output class A power of the A1.
You should be able to pick up a pair of FB1,s for £400 - £600 and FB1+ for £600 - £900 , the only major difference between the two is the original has a metal dome tweeter and the + model has a fabric dome , either will be a massive step up in sound quality .
The MF A1 is a lovely little amp and if you have a good one ( build quality was an issue ) it would be hard to significantly improve upon with a £1000 pound valve amp .
The Marantz is a good Cd player and again you would have to spend a lot more to get a better performance .
The Van den Hul interconnects if they are one of the non metallic conductor variety should be discarded and replaced with a good quality inexpensive copper interconnect from somewhere like Maplins or similar IMO, but try one for yourself and see if like me you think it is an improvement .
Here is some info on the PMC FB1 speakers , just click on the link
http://www.pmc-speakers.com/product.php?mode=view&pid=30
And a review
http://www.positive-feedback.com/Issue25/pmc_fb1.htm
Hope this helps let me know what you think , Electro.