Not sure if a step-down transformer will degrade the sound with the particular preamp, but I would expect that there is less likelihood of degradation using a transformer with a preamp, that draws an essentially constant amount of current all the time, than for instance with a Class AB or Class D power amp, where the current draw fluctuates widely with the dynamics of the music.
A transformer might even improve sonics, by filtering out high frequency noise.
Also, I believe the AC in your country is 50 Hz. Although that most likely won't be an issue, you would want to confirm that the preamp you select is rated for 50 Hz as well as 60 Hz operation.
Regards,
-- Al