Do you actually need a buffer amp?
It's quite possible the Pioneer preamp will drive both the Pioneer power amp and the cable to the mixer input. As for gain, surely the mixer has enough of that?
All you need to do is make up a "Y" cable with three pairs of connectors. Maybe a little plastic box and a tag strip from Maplins will make a reasonably tidy job of tying the three pairs of cables together. Or use a 2-into-one adaptor

At home I have a stereo amp with "tape rec" outlet which feeds cables all over the house, to 3 different destinations (one of which is a mixer, the other two are consumer stereo amps in different rooms) and it all works fine. Some of it even goes through CAT5 wiring...

Or maybe I've misunderstood the problem.

Anahata