Well here's a practical suggestion......Instead of doing a spondee test with a recording of a professional speaker, or your own voice, how about let's get spondee recordings of people who have typical voices? Professional speaker types have voices that are too easy to understand. Not everyone has a professional speaker voice!
Oh, and I add my voice to the din...... Why is it that most people just see this as a homework assignment thingy, but don't hang around for more intereaction? One would think that service providers etc would be more interested in interacting with the population they are gonna be serving!