May use a step down transformer for HI-End ?

You have a bigger problem than what is mentioned above. You need 60Hz for US equip - in Europe it is 50Hz only. A transformer WILL NOT do this conversion unless special circuitry is added to the transformer (called a cycloconverter). So, check to see if what you have will also convert the frequency. I think you may need a product specifically made for your application. However, there is also a chance that some American equip will work with 50Hz. I would look into this very very carefully before plugging anything into the socket.

You don't have to worry about the 60hz vs 50hz problem, unless you are powering a motor (turntable using an AC motor). Also might be a problem using a tape deck with AC motors. Most equipment, including those with motors, use the AC current and convert it to DC, even for the motors. I ran my Alex TT on 50Hz 110V (converted from 220v), with no problems, because its moter is a DC motor.

Most U.S. electronics are UL listed for 120 volts from 50 to 60 hz. UL listings for computers, electronics, hair dryers, etc., take into account travel to 50 hz countries and, accordingly, manufacturers design power supplies to accomadate frequency ranges as vast as 50 to 400 hz.

Lower frequency power will mainly affect motors, i.e. your hair dryer will run slightly slower at 50 hz but still put out the rated wattage. But today's non-motor electronics and audio equipment will operate at 50 hz. As long as the transformer can handle the power, it'll work fine without a converter

As always, give the manufacturer a call just to be sure. Some equipment have jumpers to convert from 120/60 to 230/50 (easy to do) so all you'll need is the euro wall plug adapter.

I moved from the States to Paris 18 months ago, and was faced with the same problem (110V gear running on 220V). Fortunately some of my equipment (Quad and Revox) is multi-voltage, no problem there. The rest is 110V equipment, Including two 200W/channel power amps. I simply purchased a 2kW 220V/110V line stabilizer transformer. You can purchase the equipment from www.eastwestintl.com or www.appliancesoverseas.com.. The other thing to remember is, HiFi equipment in Europe, especially high end gear, is a lot more expensive then in the USA. Bottom line, bring your equipment with you and use a voltage stabilizer step down transformer to run it.