Thanks for everyone advice.
Hi, Minorl, it's not only low voltage problem happens in my house, but also the voltage will fluctuate very much sometime. Last night (~10pm), I measure the voltage and it keeps swing among 208V-212V-218V-206V-212V..... it rises and drops, and then rises and drops..... it is really unstable.
I already write to the local utility abt this unstable and low voltage problem, and they are carrying out on-site inspection. They also apply a "voltage recorder" device to my house to record the voltage data for a week, but they haven't given me any result found. So, I think they are still in investigations. After 1 or 2 more weeks later, if I still get any reply, I will write to them again.
In the meantime, I try Stavol transformer (made in Japan), which is also a stablizer and able to keep 220V output. I measure the output from Stavol and it's 222V-223V. The soundstage focus improves and the sound is more lively, like a person in good spirit!! I also try Equi=tech Q1 (not sure the model name but the smallest model in the toppest grade) before, and I got the same result. However, I find that the sound, is somewhat "artificial" (not natural enough). I don't know how to describe it. Compared with plugging directly to powerbar or wall sockets, the "step-up" transformer will make the sound not natural enough. Plugging into powerbar or wall socket gives me a kind of "relax" feeling.