Update:
Still running errors free at 750M. Hashrate stays even but Zencash mined per day has dropped a little to 1.35 Zen a day. Probably due to new Z9s coming online. Getting closer to ROI.
It would be interesting and helpful to the community if one of these days you report to us if and when your Z9 ASIC dies. I would certainly find that useful information and reward you with some merit or some other way if possible.
Knowing the limits on these machines and why bitmain only shipped with lower default settings is a intriguing.
1 of my Z9 suddenly died, it was running at 750 MHz for weeks. It is now not responding after power up, no LED activity or fan spins. Tried with different working bitmain psu with no luck.