Thank you for the thoughtful reply and helpful information.
Many have said the 10" drums should be perfectly adequate, which is why I've tried to get them working right. Your method makes sense, but so did all of the previous suggestions. I'm scared to death I'll throw another $200-300 at it and still have issues.
Sigh ... the joys of old cars!
Well, they're still cheaper to work on than new cars, where some kid with a PhD in Computer Science plugs it into a $50,000 machine at $200 an hour with a 1 hour minimum, just to tell you that you need a new air freshener...