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I have no knowledge of x-aggregator, I do know though that you definitely can subscribe to individual topics with "Planet". Yes it's a bit of setting up, but not that hard. I create a Category first, then add the individual RSS feeds to that category. I activate the Category block for Planet, I click on Category A, all the RSS feeds or the one RSS feed I added to that Category is now the only topic/Category on that page. If I go to the bottom of the page there is the three Feed types these links >>> RSS | RDF | ATOM
I then either just right click RSS and select copy link location, or click on the RSS feed and a page comes up in my browser (FF3) I can get the link from that as well. So once you have the link from the Category you want you paste it into where you want it, use a neat little RSS icon to make it look like you know what you are doing.
Have a look at what I set up for one of my sites, have a look around, click on the categories in the top left hand side, check them out, look for the RSS | RDF | ATOM links click on them see how they work.
Planet ExampleI am fairly sure you can do what you want with planet, YMMV ;o)
Edit: Thought I shoulad ask what module/s you want to get the feed from, I guess not all modules are setup the same way. I know with Article, newbb, for example, the feeds can be gotten from Categories, topics etc. Maybe your problem is the module you want to get the feeds from. Just a thought ;o)