First, how do you know you have negative air flow (i.e., internal air pressure)? The way I check is to use a small source of smoke, like an incense stick or maybe a cigarette. Move that slowly around the case where there are any small cracks and see which way the smoke flows. That tells you the direction of air leakage.
Next item: check the filters on your intake fans. They need to be kept clean. Although they DO reduce air flow slightly, clogged ones are much more restrictive!
Next, let's review how the fans are being powered and controlled. The NZXT website for that case says it includes at the top and rear some model FN V2 fans, which are of the 3-pin design. So, check whether this is correct - do each of those fans have THREE wires from the motor, ending in a female connector with THREE holes? If that is true, that type of fan can have its speed controlled ONLY by a mobo fan header that uses the older Voltage Control Mode (aka DC Mode). IF a 3-pin fan's header is using the newer PWM Mode, they will always run full speed. So check in BIOS Setup for those two fans particularly whether the header is set to use DC Mode so it CAN reduce its speed as needed.