Windows 7: Audio device reverts to default.

Looked through a bunch of threads, but can't seems to find this exact one. I think this must be something minor.

1. The PC:
Purpose: HTPC. Asus M3A78-EM mobo, Athlon64 X2 Kuma 7750, 2GB Corsair DDR2-800, HIS Radeon 4670 1GB. Windows 7 RC 7100 32-bit.
For the 4670 sound, the Realtek 'ATI HDMI Audio' 2.28 installed. For onboard mobo sound, whatever Windows 7 pulled in automagically.

2. The hookup:
Case front audio ports 'HD Audio' connected to mobo FP_Audio header. The 4670 to a Sony STR-DG920 AVR through a Gefen HDMI Detective Plus.

In the control panel, only devices I left enabled are ATI HDMI rear output and 'Headphones' via Realtek HD Audio. The ATI HDMI is configured to 7.1/small speakers, set as default device, and works just fine. When I hook up headphones to a front port, default device switches to headphones, and output mode switches to stereo.

Problem: when headphones are disconnected, default audio device switches back to ATI HDMI, but the settings are wiped - that is, is is set to stereo, etc. Have to manually reconfigure it back to 7.1. Very annoying - any way for an audio device to keep its settings? Thanks.