how do I get prove to report 6 (assertions) rather than 2 (subtests)

You don't :-)

The point of subtests is so that the harness treats them as just one test regardless of how many components there are. From the Test::More doc (with my emphasis):

"subtest()" runs the &code as its own little test with its own plan and its own result. The main test counts this as a single test using the result of the whole subtest to determine if its ok or not ok.

If you want your harness to count everything inside the subtests, then just don't use subtests at all and promote all your tests to the top level.