Ampex 456 and 911 tape have very similar bias characteristics. I actually only found about 3/4 to 1 db of difference between their outputs. For the past couple of days I've been working on the tape machines in all three of my rooms and some machines are set up and biased for 456 and some are set up for 911. I would get the machine and send a series of tones from 30 to 22,000 hertz through it and check its response. If it's no more than + or - 2 db at 15 ips on playback, I would just go with it. When I'm done working on my own machines, they're usually within + or - 1 db up through 24,000 hertz at 15 ips. High end Ampex machines are notoriously stable.