Is it possible to use a AV-receiver output for an "active" speaker?
OK, you have speaker-level outputs.

If the 5.1 amp has "pre-out" use that.

Otherwise you need an adapter like this.

You can make it yourself, it's just a resistor divider, dead simple:







simulate this circuit &ndash; Schematic created using CircuitLab

HOWEVER if your amp is bridged, like some of the recent class-D ones, then you're out of luck because the "ground" speaker terminal isn't "ground", rather it is the output of the second half of the bridged amp. You should check that before doing anything. Just use a multimeter continuity tester to check the speaker "-" terminal connects to the other speakers "-" terminals for instance.