[Boulevardier]

" I agree with you, that unless the heads are open circuit (seems unlikely) that should get some sort of sound out of the amplifier, so subsequent tests will be very interesting (for me at least)"

When you get back to the heads issue, what might be a useful test (with a reel of blank or unwanted tape) would be to set the machine running on replay, and to bring your defluxer close to the head gap. Even with tape running over the head, the field of the defluxer should get through the gap to the gap and give you a hum. That running test should eliminate the possibility of head-switching issues killing the signal.

Mike