Receiver Question Pre in / Main out

I see some of the older Receivers have rca's on the back one pair says pre in and the other says main out. And there is like a jumper between them .. Can that Receiver be used as a preamp if jumper is taken out? And a amp replaced into the out put or input ?
you betcha.
Are you sure its not MAIN IN and Preamp OUT?

Typically, that jumper links the pre-amp (output) section of the receiver to the power amp (input) section.

So, if you want to drive a separate power amp, you would connect an outboard power amp to the pre-amp OUT connections.

If you wanted, you could also use the receiver's power amp section separately by connecting an outboard pre-amp to the power amp IN connections.