The M504's are going for $450 on ebay. So is the other amp worth $900? I just checked ebay for the Accoustat, and there is a listing for a TNT-120 right now. Is that the amp you are talking about? Watch it and see what it goes for. You can get a pretty nice two channel amp for $900 clams.
Hell, sell those 504's on Ebay, (I owned a 506), go to audiogon, and get a hot amp. There are two channel ss amps out the yingyang there. For under a thousand you can get an Aragon, Sunfire, BelCanto, Naim, Linn, etcetc.
Just my two cents, but that accoustat doesn't sound like a bargain.