^^ What he said.
If you have a pair of very nice speakers with inputs like that, I'd expect you would have a stereo amplifier around the place to drive them. Get a set of RCA to RCA leads, plug one end into the interface, and the other into an aux input of some sort on the amp.
The fact is that they came with an old radio which I suppose it had an amplifier or whatever inside. Are there any affordable options other than an stereo amplifier?
If you have any thrift stores nearby take a look there for an used stereo receiver. The one near me usually has a half dozen or so at about $25 or less. Sony, Denon, Pioneer are a fwe good names.
+1 to that. You can get a good, or even great, stereo amplifier for next to nothing these days.
I've had a few Denon, Marantz, and Yamaha.
If you don't have something suitable in your own attic/garage, I bet you know someone who does!
I still have an old analog tuning Marantz that I used years ago to drive a pair of speakers with my old keyboard setup. Probably a few other old stereo receivers stashed up in my barn as well. I need to do some cleaning out