I have an isomax, formerly airforce2. They are powerful and good fans. The issue I have with them is they come with a built in electronic speed controller, although the manufacturers stated you can turn it to full and hook up a variac speed controller they do have a little hum.
I have had mine on for nearly 8 years 24hours a day. So they are a quality fan.
Here is a fan test of the S and P td silent, if you look at all their vids they test all major branded fans.
At the end of the day the best option is to buy a big fan and have it run low speed using a variable transformer.