Vintage and modern hookups

You want to connect speakers to inputs?  That must have been some of Belushi's best.

I used a Phillips RV450 Surround Processor thru the tape loop with a Sansui integrated years ago and it worked great. It has it’s own speaker terminals. There are a couple on eBay right right now for cheap. Assuming you are looking for surround sound and not just another pair of little speakers plugged into that vintage amp.

Far as I can tell it's a Denon solid state AM/FM multiplex am/FM MX-990 A! I'm wanting to hook up my Polk Audio surround sound speakers with an Energy 8 in. Powered sub! 

It’s a 60s unit. 4 watts a side. RCA speaker connections. Sure you can use it. Hopefully the input won't overload with the high output from CD/DVD. Feed CD or PC audio into the tape input and make up speaker leads with RCA plugs making sure not to short the wires.
Don’t over drive it or you could damage the Polks with distortion or worse.
Hope you didn’t pay much for it though!