I think that your solution will be similar in nature to one [URL="http://www.monarchforums.com/showthread.php?t=2804#article40"]posted in the February 2009 Monarch eNewsletter[/URL].
Instead of trying to build sums and counts simultaneously in a Summary, which as you now know just won't go well, go back to the Table window and build some custom Count calculated fields for your summary to use.
You'll need a count field for each aging bucket: CurrentCount , 30Count, 60Count, etc.
For each build a formula similar to:
changing the field to get the bucket count right.
Now in your summary you'll be able to add CurrentCount, 30Count, etc., alongside your aging bucket sum amounts.
I haven't fully built it out myself, but I believe that this would work just fine.