Ralph,
I just got done with my cutover of about 100 69xx series phones and the customer wanted to reuse their old analog headsets for all of them. What I ran into is that the phone doesn't always recognize the headset and some of the 6920's headset ports seemed to be defective, but in the end I was able to make all of them work without the amplifier.
When you plug a headset into the headset port you should see the headset symbol in the information bar at the top of the screen next to the Network icon; if you don't it probably won't work. Another thing I found out is that you can go into the settings by pressing the gear button and go to Audio scroll down one and it will give you some Headset options.
I hope that helps you out at least a little bit.
Thanks,
TE