Advice wanted on Kennedy 9600 and PDP11/23

Greetings: I have a Kennedy 9600 tape drive (cream and black, horizontal loading) on an 11/23 via a third party card. I have always used the 1600bpi setting, but now have reason to read some old 800bpi tapes. The operating system is RT-11 v5.04, FB. I'm not sure if the drive supports 800bpi as alas I have no docs for it at all. My crude test was going to be a backup of du1 (the floppy) to ms0 (the kennedy) at PE setting and see how long it took and count the tape forward movements. Then do the same with the drive set for NRZI and see if it takes longer and generates more tape forward movements. A "backup/dev du1: ms0:" set at PE works fine. The same command with a /ver option also works fine. However, when I set NRZI, it looks like its backing up, the tape cycles much more slowly forward, but eventually I get an RT-11 "output error". If the drive really doesn't support 800bpi, I would think it wouldn't even try writing NRZI. So - my question is (given the above info) - is the drive broke but only for 800bpi or am I doing something silly? Thanks in advance! jay West