Subwoofer output

I am looking for a way to add a mono amp and center channel speaker to my Hegel H-390 any thoughts on how to get a mono signal to my little XTZ A2-300 ?
Look at how they do it on a LOT of the old preamps. Mac takes the right and left outputs uses a 47K resistor into a RCA for a center channel output. That’s a point to point C20 valve schematic.

C20 preamp schematic - ›

I’ve listened to mono but its been 30 years or so.. Center mono without filtering like a SST (analog) Trinaural Processor, kinda suck to tell the truth.. 

Trinaural processing is pretty cool though.