Thanks for all the advice. In fact, I have learnt from other sources it is better to keep the original Sovtek and to buy matched ones from Herron in case of replacement. Regarding Tweety's advice, I will use the Van Dan Hul's Mainsstream AC cord to see if there is any improvement.
By the way, I am matching the Herron with a pair of Passlabs Aleph 0 monoblock and the combination works wonderful. What I am thinking of is to replace the speaker with the JMLab Micro Utopia.
Any opinion or advice is welcome.
Hififan