Is it possible to reduce the power from one of these machines to 400 Watts?
What do you think and which is the best machine?
If you want to see the difference you can check this site https://www.asicminervalue.com/
and then check the power wattage on both t9+ and s9.
The big difference is the t9+ is consuming more power than s9 look at the profitability s9 is giving you more profitability compared to t9+.
About reducing the power I think you can't reduce them to 400w just the other said removing 2 board may help to reduce power wattage and you can reduce the frequency to drop down the hash rate and power consumption but only works on s9 I don't know if can work with t9+.
You can find the guide here https://bitcointalksearch.org/topic/how-to-overclock-your-autotune-antminer-s9-3239820