great receiver, I have restored one sa6400x and running it with four stacked speakers, SB510 and SB610 (horn), from the same year and catalogue. One further sa6800x is on my bench now, power amps are damaged and the relais protection kicks on.
Watch out not short the speaker wires, but if you do, have a pair of spare speaker fuzes at hand. I have many big receicers, maonstwr sansuis, etc. and it is my most loved one, very good sound for 1973 receicer, and pure joy just to look at it, when turned on. I posted some pics in another thread. Enjoy.