If you decide to try autotune, remember these two parameters: Powerlimit, and Target Temp. By default target temp is intentionally 89°, this probably makes the fans go even slower if left alone, but you can lower it if you want. And powerlimit indirectly tells it how much power it can use, and thus not exceed a certain speed voltage combination. With autotune, bosminer+ controls chip speeds individually.
You should wait until it shows "stable" before measuring power and noise. Good luck!
Autotune may let a few errors if it considers the hashrate to be more efficient even with some, so don't worry too much if you do see some, as long as the resulting hashrate / watt use is good. People have achieved even 69W/T so you should aim for something at least in the 70ies, 91 feels a bit too high... Oh but i guess you need 240v and not exceed 80% PSU capacity.
Hmm I have switched out fans and the psu's can do 1200 on 240 and 900 or 120
but with 2 boards I should set 300 watts and see what happens.