Is it OK to connect Audioengine active speakers to my stereo receiver or integrated amplifier?

Because Audioengine active speakers already have an amp built in, you never want to connect it to the amplifier output of a stereo amp or AV receiver that would typically connect with bare speaker wire or banana plugs. If you want to use an existing AV receiver as an input switcher and that system has preamp outputs (these would typically be RCA), you could connect these to the RCA inputs on your Audioengine powered speakers.