Epson says they are different "Beasts" so will not accept this as meaning anything. The P10000 comes with an Internal Print Server (hard drive) and I wonder if that is the difference.
Hard to say. The p10000 is a 300/600dpi based device as opposed to the p9000 which is a 360/720 dpi device. Not sure how it handles the fact that it receives a 360 dpi file and thus downscales to a 300dpi file. Of course it’s also designed for high production print shops and thus the internals are probably significantly more powerful, and might be doing some of the processing that Epson depends on users computers doing with other printers.
I resent the job at 8bit and it printed
I’ve never seen any visual difference between a file sent at 8 bit vs 16 bit. theoretically it seems possible and I suppose if you hunt for it side by side you might find it, but the results at 8 bit should be basically identical. This might also an anomaly with the file, would be curious if you can print another file of the same size. Also not sure how much ram your system has, but the symptom you describe is rather odd. I have seen it a couple of times and it’s when the computer choked on the data because the file size (mine struggles with an 84” at 360dpi from LR, can only print it at 8 bit. Might be a relationship of RAM and even affected by something in the OS.
One other thing I might add, although you say it’s a 400mb file, if you are telling LR to send a 360 dpi file to the printer that is 76x38”, at 16 bit you are dealing with a 2gb file, not a 400mb file. This is definitely getting into the area that LR can choke. I’m not sure how Capture one is handling this, but it could be relegating the resizing to the driver/OS. This would be the same as turning of the print resolution checkbox in LR, so LR would send the file but not actually resize it first. My guess would be this would print from LR, but the end quality might suffer because the output sharpening wouldn’t be correct and because LR seems to do a much better job at resizing the image up than whatever resizes the image without LR doing it first (OS or printer driver, seems to be a mystery).