There are a couple of ways to assure a constant voltage supply to your enlarger. The first would be to use a buck and boost transformer. The second would be to use a variac capable of supplying over supply voltage on the outputs to the load. The first is self adjusting and self compensating. The second is user adjustable.

It is important to recognize the conventional transformers are proportional devices. So as input voltage is varied so is output. Second transformers are amperage sensitive devices. This is due to the size and number of windings in the design. Therefore if your power supply is sized for 600 watts at what voltage is that calculated? If that is sized for 600 watts at 24 volt then you have no need to resize your lamp. In fact by supplying lower then design voltage you are placing less demand on your transformer then if you were supplying the proper voltage. For light output this is not a good thing since your lamp is emitting 200 watts rather then the 250 design wattage. As previously mentioned this will cause a color shift which is only a problem in color printing. It should not cause much problem in black and white printing.

The second thing to recognize is that there are two types of electrical load. The first is inductive and is characterized by such devices as transformers, coils, and motors. The second type is resistive and it is characterized by such devices as resistance heating elements and incandescent lamps.

The hum that you are hearing is probably due to the characteristics of your cooling fan motor. Not only are you supplying a different voltage frequency but also a different voltage.

There is another way, possibly, of solving this problem. You can seek to find a fan motor that will operate on 50/60 hz (either frequency). If this is a fan of the type used in cooloing applications on devices such as computers, there are probably fans available to meet your needs.