what do AC/DC input coupling on old Marantz amp do

I have Marantz SM-1000 amp and SC-9 pre amp.I do not know
which input to use or what is the difference?
DC coupling should supposedly result in better bass response BUT it can also result in damage to your speakers and amp if your preamp is "leaking" DC. Try it both ways and see if you hear a difference. If you don't hear any difference at all, run it in AC mode for the extra protection. I would also add that you should first hook it up in DC mode and watch your woofers very carefully. If they move out ANY distance and stay there when there is no signal applied to the amp, kill the power to the amp RIGHT AWAY. This means that you would have to use the AC inputs on the amp OR the amp itself is leaking DC. Sean
Let Me know if you ever decide to sell! Thanks Ronnie