I don't agree that this neccesarily indicates a problem.
I had the same experience you describe with my 750 two years ago. Here's the thread in the 750 forum:
http://ubb.outlawaudio.com/ubb/Forum2/HTML/000012.html Basically Scott in Outlaw CS indicated that the sound described in my post is normal capacitor discharge. The discharge is only audible because the 750 does not have circuitry that cuts the output to the speakers on power off. Outlaw Support, Parasound Support and Sonus Faber support all agreed that this is not harmful to the speakers in any way.
I assume that the 770 has a similar design to the 750 in this regard (no 'protection circuit'); I would call Customer support to confirm this if I were you.
Hope this helps,
Tony.
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Outlaw 976, Outlaw 7700, Pro-Ject Phono Box S
Sonus Faber Domus Grand Piano (F&C), Niles HDFX (Surr. & Rear Surr.), Outlaw LFM-1 Plus, Velodyne SMS-1
Sonos multi room audio
Video: Sony KDL-46V2500, OPPO BDP-103, TiVo Premiere XL4
2-channel: Outlaw RR2150, SF Concerto Home, Outlaw LFM-2