I wouldn’t go Bluetooth unless you can’t hear the amp, Russ. I find the latency a bit too much, makes the keyboard feel a bit sluggish. And wouldn’t using the headphone out defeat the built-in speakers? If not , maybe you could use that. I record my BK-9 off my headphone outputs and it’s as clean as a whistle, mind you I don’t run the volume at much more than 50%. I can’t speak for Yamaha, but nowadays, most headphone outs are pretty clean until you get to higher volumes…

How well it collapses to mono is definitely an issue if you want to use a stick and sub type amp (which are awesome for creating an even volume front to back of the room) so it might be worth checking before buying. Most stereo sampled pianos change tone fairly noticeably if you run them mono… How much it affects you might vary. Some don’t mind, me, it drives me bonkers!