I guess $200 isint a bad price for a good 717, its not a real deal. If you search ebay most of them are ending at around $180-200. I have a AU-517 which doesnt work too well, it needs to have some parts replaced but when it does work its a nice amp. The sound is a little lean compared to my NAD amp but i havent ever tried my axioms on it.
Honestly i think that it will leave somethings to be desired if you hook it up to the M80's because they would probbly sound better with a better amp but for 200 dollars i doubt you will be able to get anything built as good as those old sansui's
As far as measuring DC offset of the amp you can go to this thread:
http://www.audiokarma.org/forums/showthread.php?s=70356b787ba6bcfa1f128c3b8f3d81a2&threadid=5634
and read how to do it. it will give you a good idea of what kind of shape the amp is in electronically. Luckly you dont have to open the cover or know alot about electronics to do it, so you can measure the amp before you decide to buy it.
