Perhaps this might help, I am an Electrical and Electronic Engineer by profession and this is drawn from my personal experience of using a Step-Down Transformer with HiFi audio equipment.
Let us get some fundamentals in place first.
Electricity is all about the flow of electrons in wire. "Voltage" is a measure of how hard the electrons are pressing to get through--it's like water pressure in a pipe, or like the rotational force in our car's wheels. "Current," measured in amps, is a measure of how fast the electrons are flowing--it's like the gallons-per-minute flow in a pipe, or the feet-per-minute rate of spin of the tires on our car. Total power delivery, in an electrical circuit, is measured in watts, which are simply the volts multiplied by the amps; in the same way, the total power delivered by the car in our example is the amount of rotational force delivered by the wheels, multiplied by the speed of rotation. A number of watts may represent a very high voltage with relatively low current (such as we see in high-tension power lines) or a low voltage with very high current (such as we see when a 12-volt car battery delivers hundreds of amps into a starter). <p>
To all this let us add a new dimension. This dimension is called 'frequency'. Unlike direct current (DC) that flows in one direction all the time, alternating current changes direction (represented as +ve and -ve) at a predetermined cycles per second. In the US it is 60 cycles per second or 60Hz. In EU, it flows in 50 cycles per second or 50Hz. 50/60Hz in Japan.
How does this affect us? Well most AC equipment such as tube lights, heaters, and motors have been designed to work with these cycles. Are you aware that a tube light flickers (comes on and off) at the same frequency as the AC current?
Step down transformers, as with any transformer, are usually around 96% efficient. They can never be 100% efficient as there is loss of energy and current in the windings. But the catch is most step down transformers do not worry about frequency. So if you take a step down transformer and convert 230 volts to 100, what you will get is 100 volts but at 50Hz.
Now we come to the best part.
Your audio equipment should not be affected at all. Why. Simply because all audio equipment work on direct current. Power supplies inside the audio equipment contain a TRANSFORMER which converts the mains AC supply to a safe low voltage AC. Then the AC is converted to DC by a bridge RECTIFIER but the output is varying DC which is unsuitable for electronic circuits. So the next step is SMOOTHENING of the voltage. This is performed by a large value electrolytic CAPACITOR connected across the DC supply to act as a reservoir, supplying current to the output when the varying DC voltage from the rectifier is falling. Even this will only supply DC current with small ripples in voltage that yet cannot be fed to the electronic circuits. So a last step is a REGULATOR. This is usually an IC that consists of a zener diode regulator and resistors. Irrespective of what voltage is fed to the diode, it always sends out constant voltage that can be fed to the electronic circuits.
Since the HiFi equipment does an internal conversion from AC to DC and from high voltage such as 100/120/230 to somewhere around 5 to 25 volts, most equipment can, by themselves, perform well with mildly varying input voltages (voltage fluctuation). So if you do put a step down transformer between your power supply and the equipment, it should not make too much of a difference to the equipment. If the frequency is not properly managed, highly sensitive parts such as DC motors (in a CD player) could be affected. This is where PSRR of the equipment's power supply plays an important part.
Both of my Pre and Power Amp were purchased from new in Japan and used there for about 18 months, where I lived and worked for few years.
After moving to europe, I am using them with a 2Kva step down transformer. Never noticed any impact or degradation with the sound quality not even fractionally. The key here is to use a Large enough Step Down Transformer. The power draw on my Pre and Power Amp are about 444va (34 + 410) or watts. I am using a 2Kva Transformer which is rated 5 times more. If you are still unconvinced, they can easily be converted to work with EU Power supply, though its not very obvious, someone who is handy with a multimeter can easily find the 230v primaries of the internal transformer and hook them up to the mains connection and dont for get to change the fuse on the circuit board next to the AC in.
Hope this helps to bust some HiFi myths