No, you can't do that. The speaker level output of the receiver will melt the line level input of your powered speakers.
What you can do is use the TAPE OUT of the receiver to an adapter (RCA to 3.5MM stereo) then you can plug the tape level output into the powered speaker input.
I have the same issue. Received with my rocketfish a 3.5mm -> rca adapter, but get no sound. plugged in the rocketfish usb to my laptop to test the unit, and ...terrific sound. Is there something failed with the tape-out or maybe it can't be used like this? my understanding is that 'tape out' is used if for ex: i wanted to record off the 'tape in', i'd plug in a recorder and this could get the signal.
just noticed that you mention from 'tape out' to powered speaker. with the rocketfish you're going from tape out to a sending unit, then to a powered speaker. maybe a problem?
Also read that there needs to be an amplifier in what would be my rocketfish sending unit since the signal put out by tape out will be minimal. I believe the rocketfish does amplify....any thoughts??