I think that your approach is correct, however if you are getting the same results everytime then it would indicate that you are not triggering a new set of results on the results page. How do you have your DataAssist Search server behavior set up on your results page currently? Look at the trigger, rather than being set to something like the posted submit button it should be triggered on your URL parameter, productsubcategoryID. This would look like this in the code:
So long as you have the trigger set like this and have the search criteria properly set to look at the subcategory column where the subcategoryID is equal to the $_GET['productsubcategoryID'] value it should return the results for that matching sub category. If you have any further trouble with this post back with a copy of your results page in question in a zip archive and include an example link that I can check on it with.